The Anatomy of the Male Reproductive Tract of
the Cotton Rat (Sigmodon hispidus) and
Comparisons with Other
ANDREW ANTHONY ARATA
A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL OF
THE UNIVERSITY OF FLORIDA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
A debt of gratitude is expressed to the numerous people whose
help was essential to the completion of this study. Drs. Richard
Van Gelder and Sydney Anderson, American Museum of Natural History;
Dr. Emmet T. Hooper, Museum of Zoology, University of Michigan;
Dr. James N. Layne, University of Florida; Messrs. Dale E. Birkenholz,
Charles W. Myers, William 0. Wirtz III, and Larry Ogren all provided
specimens of various genera for dissection.
A special debt of gratitude is due to Mr. J. Howard Hutchison
for his time in preparing the drawing of the baculum of Sigmodon
hispidus, and to Dr. Clayton E. Ray for his stimulating conversations
Advice and criticism from the following persons is acknowledged:
Drs. Archie Carr, Henry M. Wallbrunn, Carl D. Monk, and Robert M. De
Finally, a sincere debt of gratitude is owed to Dr. James N.
Layne for his advice and counsel as committee chairman and for
indispensable help in preparation of this dissertation.
TABLE OF CONTENTS
ACKNOWLEDGMENTS. . . . . . . . . . ...... ii
TABLE OF CONTENTS. . . . . . . . .....
LIST OF TABLES . . . . . . . . . . . . vi
LIST OF FIGURES. . . . . . . . . . . . vii
INTRODUCTION . . . . . . . . . . . . . 1
MATERIALS AND METHODS. . . . . . . . . . . 4
ANATOMY OF MALE GENITALIA OF SIGMODON. . . . . . . . 8
External Genitalia . . . . . . . . . 8
Phallus. . . . . . . . . . . . 8
General morphology . . . . . . . . . 8
Baculum. . . . . . . . . . . . 11
Functional morphology. . . . . . . . . 14
Accessory Glands . . .. .. .. 17
Preputial glands . . . . . . . . .. 17
Bulbo-urethral glands. . . . . . . . .. 17
Vesicular glands . . . . . . . . . 18
Ampullary glands . . . . . . . . . 18
Prostate glands. . . . . . . . . . 19
Testis, Epididymis, and Vas Deferens . . . . . 23
Urethra. . . . . . . . . . . . . 25
General Circulation of the Reproductive Tract. . . . 26
Table of Contents (Continued)
General Innervation of the Reproductive Tract .
Sympathetic nervous system .
Parasympathetic nervous system .
COMPARATIVE MORPHOLOGY OF TIE MALE GENITALIA
MUROID RODETS . .
Mesocricetus . .
Oryzomys . .
Oecomys. . . .
Nectomys . . .
Akodon . . .
Phyllotis. . . .
Nyctomys . . .
Peromyscus . .
Neotoma . . .
Tylomys. . . .
Baiomys . . .
Ochrotomys . . .
Onychomys . . .
Synaptomys . .
Ondatra. . . .
Neofiber . . .
Microtus . . .
Pedomys. . . .
Pitymys . . .
* p p
* p p p
. p p o
. . .*
. . .
. . .
. . .
* . .
. . .
. . .
. . .
. . .
. . .
* . *.
* . .
* . .
* p p .
* . p .
* p p p
. . . . . . . .
0 . 4 .
Table of Contents (Continued) Page
Clethrionomys. . . * # . # . & . . 75
Clethronoattus . . . . . . . . . . . . . 75
Mus . . . . . . . . . . *. . .. 80
DISCUSSION . . . . . . . .. . . . . 81
The Use of Genital Structures as Taxonomic Criteria. . . 81
Patterns of Morphological Variations in the Male
Genital Tract of Muroid Rodents. . . . . . 84
Structure of the phallus . . . . . . . 84
Accessory glands . . . . . . . . 85
Taxonomic Implications ....... . . . . . 92
SUMMARY . . . . . . . . . . . . . . 97
LITERATURE CITED . . . . . . . . 99
FIGURES. ........ . . . . . . 104
BIOGRAPHICAL SKETCH. . . . . . . . . . . . 223
LIST OF TABLES
1. List of Species and Number of Specimens (in parenthesis)
Examined. ...... ....... . . . .. .... 6
2. Summary of the Variation in Accessory Glands of 24
Muroid Genera .......... ...... ..... 86
LIST OF FIGURES
1. External genitalia of male Sigmodon hispidus . . . 108
2. Base of the penis of Sigmodon hispidus. . . . . .. 110
3. External morphology of the glans penis of Sigmodon
hispidus. . .. . . . . . .. . . . 112
4. Lateral view and selected cross sections of the glans
penis of Sigmodon hispidus . . . . . . . . 114
5. Details of the internal structure of the glans penis
of Sigmodon hispidus. . . . . . . . . . 116
6. Dorsal views of the circulatory apparatus of the glans
penis of Peromyscus, Sigmodon, and Mesocricetus . . 118
7. Details of the glans penis of Sigmodon hispidus . . . 120
8. Dorsal views of the circulatory apparatus of the glans
penis of Neofiber, Ochrotomys, and Neotoma. . . . . 122
9. Several views of the internal portions of the glans
penis of Akodon . . . . . . . . . 124
10. Several views of the internal structure of the glans
penis of Rattus . . . . . . . . . . . 126
11. Baculum of Sigmodon hispidus. . . . . . . . 128
12. Synovial joint between shaft and median distal process
of the baculum of Sigmodon hispidus . . . . . . 130
13. Baculum and associated circulatory structures of a
six month old Sigmodon hispidus that had been castrated
at about one month of age . . . . . . . . 132
14. Male reproductive tract of Sigmodon hispidus. . . . 134
15. Basic functional changes occurring in the penis of
Sig~odon hispidus at erection . . . . . . . 136
16. Cross sections of the preputial gland of the Sigmodon
hispidus. . . . . . . . . . . . 138
List of Figures (Continued)
17. Cross sections of urethral glands of Sigmodon hispidus. .
18. Diagrammatic longitudinal section through caudal
region of male Sigmodon hispidus. . . . . . . .
19. Cross section of the vesicular gland of Sigmodon hispidus
20. Cross sections of the ampullary glands of Sigmodon
hispidus, . . . . . . . . . . . . .
21. Cross sections of the median ventral prostate gland of
Sigmodon hispidus . . . . . .....
22. Cross sections of the lateral ventral prostate gland of
Sigmodon hispidus . . . . . . . . .
23. Cross sections of the dorsal prostate gland of Sigmodon
hispidus . . . . . . . . . . . .
24. Cross section of the anterior prostate gland of Sigmodon
hispidus . . . . . . . . . . . .
25. Diagrammatic representation of the duct systems of the
accessory reproductive glands of Sigmodon hispidus in the
region of the prostatic urethra . . . . . . .
26. Testes and associated structures of Sigmodon hispidus . .
27. Distribution and variation in the course of the spermatic
artery on the surface of the testis in 7 pairs of
Sigmodon hispidus testes. . . . . . . ... .
28. Cross sections of the testis of Sigmodon hispidus . . .
29. Cross sections of the caput epididymis of Sigmodon
hispidus . . . . . . . . . . . .
30. Cross sections of the corpus epididymis of Sigmodon
hispidus . . . . . . . . . . . .
31. Cross sections of the cauda epididymis of Sigmodon
hispidus showing region of ciliated epithelium. . . .
32. Cross sections of the cauda epididymis of Sigmodon
hispidus showing region of unciliated epithelium. . .. .
List of Figures (Continued)
33. Longitudinal section of the vas deferens of Sigmodon
hispidus. . . . . . . . . . . .
34. Cross sections of the urethra of Sigmodon hispidus
at different levels . . . . . . . .
35. Distributional pattern of the nerves to dorsal surface
of the glans penis of Sigmodon hispidus . . . .
Male reproductive tract of Mesocricetus . . . . .
Male reproductive tract of Oryzomys . . . . . .
Male reproductive tract of Oecomys. . . . . . .
Male reproductive tract of Nectomys . . . . . .
Male reproductive tract of Akodon . . . . . . .
Male reproductive tract of Phyllotis. . . . ..
Male reproductive tract of Nyctomys . . . . .
Male reproductive tract of Peromyscus . . . . .
Male reproductive tract of Reithrodontomys. . . . .
Male reproductive tract of Neotoma. . . . . ..
Male reproductive tract of Tylomys. . . . . . .
Male reproductive tract of Baiomys. . . . . . .
Male reproductive tract of Ochrotomys . . . .
Male reproductive tract of Onychomys. . . . . . .
Male reproductive tract of Synaptomys . . . ....
Male reproductive tract of Ondatra. . . . . . .
Male reproductive tract of Neofiber . . . . . .
Male reproductive tract of Microtus . . . . .
Male reproductive tract of Pedomys . . . . ..
List of Figures (Continued)
55. Male reproductive tract of Pitymys. . . . . . .
56. Male reproductive tract of Clethrionomys. . . . .
57. Male reproductive tract of Rattus . . . . .
58. Male reproductive tract of Mus. . . . . . . .
In many animal groups, great importance has been attached to the
genitalia, especially that of the male, for purposes of classification.
In the Mammalia, classical taxonomy has not considered the genitalia,
but has been concerned mainly with osteological and dental characters.
This is not to say that genital characters have been unknown; rather
that, though certain elements were known, they have largely been
ignored until comparatively recent years.
The most distinctive element of the male genitalia, the phallus
and its bone, the baculum, have been described in many mammals over a
period of years. Most early works are descriptive, and few are of a
comparative nature. A vast literature exists on the baculum of mammals.
Most descriptions are scattered, however, and a few are inaccurate.
Periodically, attempts have been made to catalog the bacula of certain
taxa or of mammals of certain geographic regions. Important works of
this nature are those of Gilbert (1892), Chaine (1925), Burt (1936;
1960), Blair (1942), Hamilton (1946), Didier (1943; 1948; 1950; 1952;
1953; 1954), White (1953), and Anderson (1960).
Although the baculum serves as a useful tool for taxonomic work
at certain levels, its use is limited by its sporadic distribution
within the Mammalia. It is not present in the Prototheria or Metatheria
and is known from only six of fifteen Recent orders of Eutheria. Further,
in certain orders (i.e., Insectivora, Primates) not all genera possess
the baculum. On the other hand, the general morphology of the genitalia
may be compared between taxa at any level, or within taxa.
The phallus and the accessory glands offer a field for comparative
anatomical study that has been poorly exploited. From the early 1800's
numerous works have appeared describing the male genitalia of mammals
in various degrees of detail. Oudemans (1892) described part, or all,
of the genitalia of 50 mammalian genera, and summarized the literature
on accessory glands available at that time. Engle (1926) incompletely
summarized the literature and gave information on the accessory glands
of representatives of 16 orders. However, despite the extent of
early work on this subject, the only order that received a nearly
comprehensive coverage was the Insectivora (Kaudern, 1907; 1911).
In the Rodentia Tullberg (1899) discussed the male genitalia of
several genera in a sketcIh fashion. Only the Sciuridae have received
intensive study (Mossman, et al., 1932; Prasad, 1957). The more recent
work on the genitalia of rodents has dealt with forms widely scattered
throughout the order. Some of the more important of these papers have
dealt with: Aplodontia (Pfeiffer, 1956), Thomomys (Gunther, 1956;
1957), Rattus (Price, 1936), Mus (a number of early papers summarized
by Snell, 1941), Microtus (Hamilton, 1941; Delost, 1955), Neotoma
(Howell, 1926), Chinchilla (Beyerlein and Hillemann, 1952), Erethizon
(Mirand and Shadle, 1953), Myocastor (Hillemann, et al., 1958; Stanley
and Hillemann, 1960). Most of these studies are descriptive, with
little or no attempt at comparison with related forms.
A recent series of papers by Hooper (1958, 1959, 1960) on the
comparative morphology of the glans penis of members of the Cricetinae
has been of a different nature, however. Hooper has demonstrated that
clear distinctions in the phallus exist within the subfamily, and that
certain genera show closer affinities to members of other subfamilies
and families than to genera supposedly belonging to the same subfamily.
The purposes of this study are to determine the extent and the
nature of the variations occurring in the structures of the male geni-
talia of muroid rodents, and how they relate to the current taxonomic
arrangement. The anatomy of the male reproductive tract of a represent-
ative form (Sigmodon hispidus) has been studied in detail as a basis
for comparison with other muroids.
MATERIALS AND METHODS
Specimens of genera received from various sources were preserved
in different fluids. Only locally collected specimens of Sigmodon
hispidus were sectioned. In specimens to be used for histological
study the reproductive tract was removed as soon after death as possible,
oriented, and fixed in formalin or formalin, alcohol, and acetic acid
(FAA). For routine histology of the accessory glands and other soft
parts, paraffin embedding and staining with hematoxylin and eosin or
Van Giesen's and orcein was employed.
All genital tracts utilized for gross study were removed from
the specimens under a dissecting microscope. Fat and associated tissues
were teased from the tracts with finely ground steel forceps. Some
specimens were dissected while immersed in ether to facilitate the
elimination of fat from very small glands.
Whenever possible, dissection under a dissecting microscope was
used for examination of prostatic ducts and microanatomy of the glans
penis. In addition, specimens were mounted in collodion and handsections
made (Carleton, 1957; Pantin, 1959). Sections of 0.5 1.0 mm were thus
prepared and stored in clove oil. Staining of hand-cut sections was
accomplished in either hematoxylin and eosin or chlorazol black. It was
found that tertiary butyl alcohol could be substituted for chloroform in
the method described by Pantin (1959).
To examine the circulatory system, arterial and venous injections
were made with gelatin solutions of india ink (venous) and carmine
(arterial), following a circulatory flush with warm, isotonic saline
injected into the left ventricle of the living animal (De Santo, 1960).
For preparations of bacula, the standard two per cent KOH
macerating solution followed by staining with alizarin red and clearing
in glycerin was used (Anderson, 1960). Some bacula were prepared by
dissection rather than clearing, a technique more tedious but consider-
ably better for detailed examination of the baculum and adjacent parts
of the circulatory system.
Ground sections of the baculum were prepared by embedding the
bone in heated balsam on a slide and allowing the balsam to cool to a
glassy consistency. The specimen could then be ground by the use of
carborundum on a glass plate. The slide was then heated, the bone
removed and reversed, and the opposite side ground. This procedure
was repeated until the desired thickness was attained.
Approximately 50 male specimens of Sigmodon hispidus were used
in this study. The drawings of the microanatomy of the glans penis
represent information obtained from over 25 dissections of this organ.
The number of specimens of other genera studied ranged from one to
LIST OF SPECIES AND NUMBER OF SPECIMENS
(IN PARENTHESIS) EXAMINED
Mesocricetus auratus (2)
Oryzomys Dalustris (10)
Oecomys tectus (2)
Nectomys alfari (2)
Tylomys fulviventer (2)
Nyctomys sumicrasti (1)
Reithrodontomys humulis (2)
Peromyscus gossypinus (4)
Ochrotomgs nuttalli (2)
Baiomys taylori (1)
Onychomys leucoiaster (1)
Akodon s. (1)
Phyllotis danrini (1)
Sigodon hispidus (50)
Neotoma floridana (3)
Synaptomys cooper (2)
Clethrionomys gapperi (1)
Ondatra zibethicus (5)
Neofiber alleni (4)
Pitymys pinetorum (3)
Microtus pennsylvanicus (2)
Pedomys ochrogaster (1)
Rattus norvegicus (10)
Mus_ musculus (5)
Total number examined. . 117
ANATOMY OF THE MALE GENITALIA OF SIGMODON
Externally, only the penile papillus and scrotum are visible.
The penile papillus is unpigmented, and the scrotum is black in breed-
ing adults. The median raphe is clearly visible, even in young animals
in which the testes are not scrotal. The scrotum is hairless ventrally
and thinly haired laterally.
The testes when fully scrotal protrude about 5 10 mm posterior
to the anus (Fig. 1), and are in contact with one another medially.
The tips of the cauda epididymides flare to give the fully mature
scrotum a slightly forked appearance. The caput epididymis is well
developed and surrounded by yellovish-green fat.
The lateral preputial glands are too large to fit within the
confines of the prepuce and encroach upon the ventral aspect of the M.
General morphology.-The phallus originates in the bulb of the
urethra. The urethra turns antero-ventrally (about 1000) and receives
blood from several sources to form the corpus cavernosus urethrae. The
corpus cavernosus is formed by the median union of the crurae which
originate on the posterior border of the ischium, along with the M.
ischiocavernosus with which they are intimately associated (Fig. 2).
The body of the penis is about 20 25 mm long, the dorsal border lying
on the ventral surface of the M. rectus abdominus (Fig. 2). The glans
penis is deflected ventrally at an acute angle.
The prepuce of the glans penis contains the preputial glands,
which consist of a small ventral pair and a larger lateral pair. The
surface of the glans is spined. The distal part (1/3 1/2) of the glans
is corrugated (Fig. 3) in the flaccid state.
Internally, the structure of the glans penis is complex (Figs. 4
and 5). The baculum lies in the middle of the glans, surrounded by
associated circulatory structures and erectile tissue. Lateral to the
shaft of the baculum are large blood sinuses, the penile blood sacs.
These are connected to the corpus cavernosus urethra, from which they
receive blood (Fig. 4). Circulatory tissue surrounds the whole distal
structure of the baculum (Figs. 4 and 5). The blood supply of the distal
structures is continuous with that of the penile blood sacs (Fig. 4).
The circulatory apparatus of the glans penis is drained by the dorsal
vein of the penis, which is formed by the Junction of large vessels
from the lateral aspects of the glans and confluent with the penile blood
sacs (Fig. 4). The circulatory apparatus of the glans penis is drained
by the dorsal vein of the penis which is formed by the junction of large
vessels from the lateral aspects of the glans and confluent with the
penile blood sacs (Fig. 6). The dorsal blood vessel is situated in a
depression on the dorsal surface of the base of the baculum (Fig. 6), and
cradled between the lobes of the corpus cavernosus penis in the body of
the phallus (Fig. 6).
The urethra terminates near the base of the median process of the
baculum. The lappet tips of the urethra lie on the broad shelf of
this median process and are separated by a ventral keel on the latter
(Figs. 4 and 5).
The penile blood sacs are connected by a muscle, M. suburethralis
penis, which lies under the urethra on the ventral side (Fig. 8).
Slips of this muscle insert upon the broad shelves of the median process
of the baculum, distally, and upon the ventral surface of the corpus
cavernosus penis at the insertion of the latter upon the base of the
baculum. Dorsally, a tendon, the dorsal tendon of the penis, connects
the dorsal surface of the corpus cavernosus penis at the base of the
baculum to the lateral processes of the distal portion of the baculum.
Erectile tissue is most abundant in the walls of the glans penis
at the distal end, becoming less well developed proximally (Fig. 4).
The surface of the glans is spined, each spine being situated in
a depression (Fig. 7). The spines appear to be formed of cornified
epidermal tissue. There are approximately 125 spines surrounding the
glans at any level near the distal end (Fig. 7). The circumference of
the corrugated surface of the distal end of the phallus in the erected
state is approximately double that of the flaccid condition.
Compared with the glans of other genera, Sigmodon. Mesocricetus,
Akodon, Neofiber and even Rattus, as well as many other cricetids and
murids (Figs. 6, 8, 9 and 10) stand in strong contrast to such genera
Peromyscus, Ochrotomys, Neotoma, and many New World cricetines.
Baculum.--The baculum of S. hispidus has been figured previously
by Hamilton (1946) and Burt (1960). However, neither of these illus-
trations is completely accurate, since some of the salient characters
of the structure are not shown.
The baculum consists of a proximal shaft, and three digitate
processes at the distal end (Fig. 11). At the broad base are two
rounded condyles to which are attached the M. corpus cavernosus. The
concavities of the base of the baculum are confluent with similar
concavities in the muscle and in which is situated the dorsal vein of
the penis and the urethra on the dorsal and ventral aspect respectively.
The dorsal vein, formed from several veins of the glans penis, originates
in the dorsal depression on the base of the baculum.
The shaft of the baculum is rod-shaped in very young animals and,
though it reaches definitive length early in development, the enlarge-
ment of the base occurs later, in the area of the attachment between the
bone and the M. corpus cavernosus penis and increases with the growth
of the muscle. The condyles in many old animals become rugose.
From the broad base of the baculum, the shaft tapers distally
(Fig. 11), terminating in a laterally flaring bulb in many, but not all,
specimens. The shaft is generally rounded (as reported by Burt, 1960),
but in some individuals appears more triangular, or, ovate. Hamilton
(1946) figures a baculum with a ". . prominent deep keel on the
ventral aspect." Burt (1960) did not observe such a keel. I have
examined over 50 specimens and, though some possess a very shallow keel,
none are as distinctive as that figured by Hamilton. Perhaps consider-
able variation exists in this character between the 34 recognized sub-
The bony base of the shaft and the proximal portion of the shaft
is cancellous in some specimens. There is a narrow medullary cavity
running the length of the shaft. The nature of this cavity is the same
as that described for Rattus by Ruth (1934): "It is not a series of
haversian systems, but it is rather one large system in which the medullary
cavity represents the haversian canal. . ." The baculum of Sigmodon
appears to have the same growth pattern as that of Rattus, with a single
medullary cavity, growth in width being accomplished by appositional
growth of subperiosteal osteoblastic activity. Definitive size is
accomplished first in length, the base developing later. Diverticula
run diagonally from the medullary cavity to the periosteum.
It is the distal architecture of the baculum that is most
distinctive. Hamilton (1946) represented this tri-partite structure as
a simple trident-shaped affair when viewed dorsally. Burt (1960) also
showed the digital processes as a single trident, but in a bit more
detail. Although the orientation is not stated in the figure, it appears
to be a dorsal view.
The median process is the most ventral and extends from the axis
of the shaft at about a 100 angle. It terminates in a dorsallydirected
hook, bending at about a 700 angle. On the proximal ventral surface of
this process are two laterally flaring shelves, separated by a small
keel (Fig. 11). Though the major portion of this process ossifies late
in life, I have not seen ossification of the shelves and keel though I
have examined some very large specimens. The two lappet tips of the
urethra lie on the lateral shelves, being separated, and in the flaccid
condition blocked, by the central keel.
The two lateral processes are in contact with one another
proximally on the dorsal side of the median process (Fig. 11).
All three digital processes will ossify with advanced age. The
two lateral processes do not necessarily ossify concurrently. Some
specimens will show both lateral processes ossified, some only one.
The site of ossification is in the central core of the process close to
the distal end.
An interesting feature of the structure of the baculum is that a
synovial joint (Fig. 12) exists between the digitate processes and
the shaft. This movable joint has apparently not been noted previously
and, paradoxically, is not in accord with the generally assumed sesmoid
nature of the bone. Neither are the facts of ossification demonstrated
in Rattus by Ruth (1934), which are apparently identical in Sigmodon.
Ruth demonstrates an unusual bony formation for the shaft in which
osteogenicc cells become active in laying down osteoid substance before
any marked differentiation of surrounding tissues can be observed an
endo-blastemal ossification." From my observations on the form and
development of the baculum of Sigmodon, I see no point at which the
development differs from the description of Rattus by Ruth (1934).
The similarities of other parts of the phalli substantiate the fact
of their similar origin.
The development of the baculum is apparently under hormonal control.
Figure 13 illustrates the baculum of a six-month-old Sigmodon hispidus
that was castrated when less than a month old. The form of the baculum
is that of a juvenile animal. Normal development of the associated
circulatory structures is apparent however. Normal six-month-old
animals of this species would have a baculum similar to that illustrated
in Figure 11.
Functional morphology.-The great morphological diversity existing
between the varied penile types in the Cricetidae prompts one to specu-
late regarding the function of such elaborate accessory structures as
possessed by Sigmodon. The simple nature of a phallus such as that of
Peromyscus would seem to be equally simple in function. The glans is
elongate and strengthened by the spine-like baculum. The urethra lies
ventrad of the baculum and is poorly supplied with erectile tissue.
The functional nature must necessarily be that of a narrow-gauge tube,
the insertion of the penis into the vagina being aided by the rigidity
supplied by baculum. In the short, squat glans of Sigmodon (as well as
numerous other genera with a trident-shaped baculum) no such simple
functional explanation seems to suffice. The operation of the baculum
only becomes apparent when the glans is considered as a morphological
unit. In all cases in which a trident-shaped baculum is present, it is
accompanied by the accessory circulatory structures previously described.
Vascular tissue surrounds the whole distal end, and the sizable penile
blood sacs constitute a large portion of the mass of the glans. The
tendon on the dorsal aspect of the baculum, the synovial joint at the
junction of the shaft and distal processes, and the sub-urethral muscle
must all be considered.
In the flaccid state the corpus cavernosus urethra and corpus
cavernosus penis are not engorged with blood, and the erectile tissue
of the glans is relaxed. The penile blood sacs are unexpanded, with but
a small amount of blood coursing through them and around the distal tips
of the baculum. An equilibrium is established between the dorsal tendon
of the penis and the tendons of the M. suburethralis penis connecting
the distal portion of the baculum to the corpus cavernosus penis on the
ventral side. The resulting position of the distal processes of the
baculum is such that the lappet tips of the urethra lie upon the broad
shelf of the median process, separated by the strong ventral keel. A
strong ventral flexure is present at the junction of the glans to the
body of the corpus cavernosus penis (Fig. 14).
In order to induce erection in the penis of freshly killed
Simnodon, the following techniques were employed. The base of the penis
was ligatured, and fluid or air under pressure (controlled by mouth
using a small diameter rubber tube) was forced into the dorsal vein of
the penis and/or the body of the corpus cavernosus penis. The penis
straightened, the glans falling in the same plane as the body of the
penis. The penile blood sacs became distended and the distal processes
of the baculum turned dorsad. This was accomplished by the relaxation
of ventral pressure due to the distortion in the M. suburethralis
accompanying the enlargement of the penile blood sacs to which they are
attached. The dorsal tension produced by the dorsal tendon of the penis
was increased as a result of the straightening of the glans (Fig. 15).
The distal processes of the baculum were forced apart slightly by the
engorging of their respective vascular sheaths. When the dorsal motion
of the distal processes is accomplished, the lappet tips of the urethra
are forced from the obstructing keel of the median process, and the
urethral orifice is unimpaired. If this dorsal flexure of the distal
processes did not occur the semen would not be afforded a free path into
the vagina. The urethral orifice is blocked in the flaccid state.
Other changes occurring upon erection of the penis are the
elevation of the penile spines located in pockets in the flaccid state
and the expansion of the glans to nearly double its size in the non-
erect state (Fig. 15). These two phenomena probably serve to secure the
position of the glans and dilate the vagina. There may be some "lock-
and-key" relationship between the dorsally flexed fork of the baculum
and the vagina of the female, but I was unable to find any such associ-
In addition to the function already ascribed to the penile blood
sacs, a further function seems probable. The usual relationship of
arterial and venous structures fail in the penis. Though I have not
been able to find them in Sigmodon, "sluice valves," and various forms
of hydraulic controls have been described in the penile circulation of
dogs (Christensen, 1954), man, and certain vespertilionid bats (Wimsatt
and Kallen, 1952). Some of these include expanded blood sinuses about
the baculum (dog and bats). Although these are quite probably not
homologous with the penile blood sacs of Sigmodon, they do seem to be
analogous. To maintain a constant vascular pressure in the penis, some
sort of hydraulic control is necessary. The "sluice valves" described
by Christensen (1954) is one method, another would be maintenance of
pressure by a sac-like arrangement operating on the principle of a
water bulb on a hydraulic pump.
Preputial glands.--These glands lie in the tissue of the prepuce
(Fig. 7). There are two pairs, a small ventral pair that is completely
within the confines of the prepuce, and a larger lateral pair that
encroaches upon the ventral border of M. rectus abdominis (Figs. 3 and
14). Each of the four glands is drained by a single duct. The small
ventral glands measure about 10 x 3 mm in greatest length and width,
respectively. Corresponding measurements for the larger pair are
18 x 5 mm.
The preputial glands are modified sebaceous glands. The large
polyhedral cells are best seen in the basal part of each unit, prior to
degeneration and formation of sebum. The nuclei are centrally located
in the secretary cells and the ducts are lined with squamous epithelium
Bulbo-urethral glands.--These paired, tubulo-alveolar glands lie
anterior and lateral to the urethral bulb (Fig. 14). Each gland is
pear-shaped and is drained by a single duct. The glands are bordered by
the M. bulbocavernosus (caudad), the M. ischiocavernosus (cephalad), and
the M. externa caudae media mediallyy). The ducts pass between the
M. ischiocavernosus and M. bulbocavernosus to enter the dorso-lateral
walls of the urethra just anterior to the urethral bulb. The ducts
pass through the tissues of the membranous urethra to enter the lumen
at the point at which the urethra turns ventrad to form the penile
urethra. Each of these glands measure about 5 x 6 mm.
The secretary cells of the bulbo-urethral glands average 30 x 20
microns. When stained with hemotoxylin and eosin the nuclei are very
dark blue and the cytoplasm light pink. The nuclei are situated at the
base of the cell adjacent to the well developed basement membrane
Vesicular glands.-The vesicular glands (Fig. 18) are the largest
of the accessory glands of Sigmodon. They are situated dorsad of the
bladder and form the most anterior portion of the tract. Not infrequently
they will be pushed together by pressure from the caecum of the gut.
The vesiculars are recurved, lumpy, and frequently lobate along the
greater curve. From the duct to the terminal portion along the greater
curve they may measure as much as 40 mm. The maximum thickness is
generally about 10 mm. Each gland is drained by a single duct, which
enters the urethra dorso-lateral to the ampulla.
The vesicular glands are covered by a thick sheath of connective
tissue, beneath which a thick layer of smooth muscle is present. The
mucous membrane is thrown into an amazing series of folds which provides
large secretary surface (Fig. 19). The secretary cells are elongate
(15 x 5 microns), and the nuclei situated at the basal portion adjacent
to the basement membrane. The waxy secretion is eosinophilic.
Ampullary glands.-These paired glands lie directly anterior to the
neck of the bladder (Fig. 14). They are closely associated with their
respective vas deferens and are contained within the same connective
tissue sheath. The glands are the smallest of the accessory glands,
the pair measuring about 6 mm in width, and about 4 mm in thickness
These are branched tubulo-alveolar glands. The epithelium is
heavily contorted, but owing to the small size of the glands, the
secretary surface is not extensive. The individual secretary cells
average 10 15 microns (Fig. 20). These glands are drained by
numerous ducts, as many as 14 to a gland, which enter the vas deferens
and vestibule of the ampulla.
Prostates.-These are the most confusing glands in the reproductive
tract. They are extensive, large in number, variable in color, inconstant
shape, and the homologies of glands designated as prostates in different
mammalian groups are not clear.
Walker (1910) described the glands that lie in the lesser curve
of the vesiculars as "coagulating" glands. He emphatically denied the
association of these with the prostatic series. The name "coagulating"
has been used since that date by most authors for these glands. Price
(1936) demonstrated that the prostates of Rattus arose embryologically
from three cords, destined to be the three main prostatic groups. The
"coagulating" are the most anterior of these three lobes, and are
therefore, prostates. Price (1936) termed these three prostatic groups
"posterior," "mid," and "coagulating." Hamilton (1941) discussing
Microtus used the term "ventral lobes" and "ventro-lateral" lobes. The
"ventral" lobes of Hamilton are however, the ampullaries, not part of
the prostatic series (as can be seen from his illustration). The
"ventro-lateral" lobes of Hamilton are the "coagulating" glands of Walker.
The difficulty involved in the naming of these glands is especially
great when comparing different genera, whose prostatic compliment may not
be the same.
The prostates are clearly divisible into three major groups in
Rattus (Price, 1936), Sigmodon. and many of the genera examined in
this study, and I have chosen to designate these groups as "anterior,"
"dorsal," and "ventral."
The anterior prostates correspond to the "coagulating" of
Walker (1910), Price (1936), and Snell (1941); and the "ventro-lateral"
of Hamilton (1941). I have not used "coagulating" because we are not
certain of the function of this, or for that matter, any other accessory
gland. I have not used "ventro-lateral" because there are ventral and
lateral prostates other than these. I have chosen "anterior" because
of all prostates these are the most anterior, and because the ducts of
these enter the anterior portion of the prostatic urethra.
The "dorsal" prostates are the same as those similarly designated
by Sneed (1941) and are the "mid lobe" of Price (1936). Hamilton (1941)
did not mention these glands in Microtus though they are present. These
glands are on the dorsal border of the prostatic urethra.
The "ventral" prostates are the "posterior prostates" of Price
(1936), the "prostates" of Walker (1910), and the "ventral" prostates of
Snell (1941). As previously mentioned, the "ventral" prostates of
Hamilton (1941) are ampullary glands. These glands lie on the ventral
surface of the prostatic urethra, posterior to the bladder. In Sigmodon.
and many other muroid genera, there are at least two sets of these.
The two pairs of the ventral prostates found in Sigmodon hispidus
consist of a median and lateral pair (Fig. 3). The former lies posterior
to the bladder. The glands are elongate and filiform. At the base of
each gland 8 10 tubules are visible, and sections of the prostatic
urethra indicate that there may be a corresponding number of ducts from
each of the glands. The glands are highly irregular in shape, twisted
about fat tissue, and pushed against the bladder or the vesiculars.
When straightened, each gland may be 15 25 mm long at maximum develop-
ment. As many as 50 75 tubules may be present at the terminal portion.
The ducts of these glands enter the ventral wall of the prostatic
urethra caudad and slightly ventrad to the neck of the bladder. These
glands are white when preserved in formalin or FAA.
The tubules are lined by columnar epithelium, the cells measuring
20 25 microns in length and 10 15 microns in breadth. The mucous
membranes are thrown into heavy folds (Fig. 21). The smooth muscle
between the tubules is not extensive. These glands do not section
easily, due, apparently, to the crystalline nature of the secretion
as it is precipitated by the fixative. The tubules of this gland are
loosely bound together by connective tissue.
The second pair of ventral prostates, equal in size and number of
tubules to the median pair lies lateral to the latter. The ducts of
these glands enter the ventral wall of the prostatic urethra lateral
to the neck of the bladder. As few as seven and as many as nine ducts
are present in each member of the pair. These glands assume a reddish
to purple color when preserved in formalin or FAA.
These glands are compound tubular (as many as 75 tubules present
at the terminal edge) glands, and closely resembles the pair of median
ventral prostates (Fig. 22). The secretary epithelial cells are more
elongate however, averaging 25 30 microns in length.
The small leaf-like dorsal prostate glands lie at the dorso-
anterior border of the prostatic urethra (Fig. 23). The colon passes
medially between the members of this pair. The glands are compact,
and bound tightly in a sheath of connective tissue. In the adult
each gland is about 7 x 5 x 2 mm in length, width, and thickness,
respectively. Each gland drains into the dorso-anterior wall of the
prostatic urethra through four to six ducts.
The dorsal prostate glands are not filiform, the tubules being
only visible upon dissection. About 15 tubules can be distinguished
in each of these glands. The tubules are of a smaller calibre than
those of the ventral prostates, and the mucous membrane is not as
heavily folded. The columnar epithelial cells are lower, averaging
15 20 microns, and the smooth muscle is proportionately thicker.
The secretion has a granular consistency.
The paired anterior prostate glands lie in the lesser curve of
the vesicular glands (Fig. 14). They consist of long filaments fre-
quently lying on either medial or lateral surfaces of the vesiculars,
or both. The tubules are large, averaging 3 5 times the diameter
of those of the ventral prostates. There may be from 25 35 tubules
in each gland of the pair. These are drained by anastomosing ducts
which lie adjacent to the duct of the corresponding vesicular gland
Histologically, the anterior prostate glands are quite similar
to the other prostate divisions. The compound tubules are larger, how-
ever, and the mucous membranes are not as extensively folded. The
columnar epithelial cells are small, averaging 15 20 microns in
length (Fig. 24). The smooth muscle is not as extensive as that of
the dorsal prostates. The secretion appears to be somewhat granular
like that of the dorsal prostates.
Testis, Epididymis and Vas Deferens
The testis of Sigmodon hispidus is a large, oval, compound
tubular gland (Fig. 26). In a fully mature individual the testis may
measure up to 25 mm in length by 15 mm in width. The tunica albuginea
is pinkish white in a living animal. The course of the superficial
testicular artery on the surface of the testis is very tortuous.
Harrison (1949) figures radiograms of injected testicular arteries for
numerous genera of mammals and compared the distribution of this
artery. The distribution of this artery in Signodon, though quite
variable (Fig. 27), is very similar to Harrison's illustration for this
vessel in Rattus. Although Harrison commented upon the possible
taxonomic utility of this artery, the course taken by it is variable
within a species and, in the case of the muroid rodents, too similar in
many genera to be of usefulness. Its pattern does seem to differ
significantly between different orders of mammals, however (Harrison,
The cremaster muscle surrounds the testis in adult specimens. The
fibers of the cremaster originate in the region of the inguinal canal
and terminate on the testis and scrotum. The fibers are directed
posteriorly and are most dense along the dorsal border of the testis.
The cremaster receives blood from a branch of the external spermatic
The testis is divided into numerous lobules separated by projections
of the tunica into the body of the testis. The seminiferous tubules of
these lobules join at the rete testis, then form efferent ducts which
drain the testis. The seminiferous tubules average 210 microns in cross
section. A vell-developed basement membrane is formed (Fig. 28).
The Sertoli cells are round to polyhedral, about 10 microns in diameter.
The nuclei of these cells are large (4 5 microns) and centrally
The caput epididymis is large, capping the cephalic pole of the
testis (Fig. 26). It is divided into eight lobules, the first of which
receives the vasa efferentia. I could only recognize two efferent
ducts in my material of Sigmodon. Mus (Snell, 1941) and Myocaster
(Stanley and Hillemann, 1960) are reported to have five to six such
ducts. The first lobule of the caput epididymis is a deep purple
color in formalin preserved specimens, the remaining seven lobules are
a light lavender. The epididymal ducts of the caput are large, averag-
ing 0.5 1 mm in diameter (Fig. 29). The epithelial lining of these
tubules is columnar, the cells up to 30 microns long. The nuclei of
these cells are situated at the base of the cells along the well
developed basement membrane.
The corpus epididymis runs along the median aspect of the testis.
In the corpus the fibrous connective sheath common to all parts of the
epididymis is well developed (Fig. 30). In a single cross section of
the corpus epididymis, the ductus epididymi is traversed 20 30 times,
so convoluted is this tube. The epididymal duct is 150 170 microns
in diameter through the corpus. The columnar epithelial cells average
23 microns, and have their nuclei located basally.
The large cauda epididymis is located slightly lateral, but caps
the posterior pole of the testis (Fig. 26). The tubules are visible to
the naked eye in sexually mature specimens, being as much as 1 mm in
diameter. They average somewhat smaller than the massive tubules of
the caput. The tubules of the cauda epididymis are lined by ciliated
columnar epithelium near the formation of the vas deferens (Fig. 31),
or by simple columnar epithelium in the major portion of the structure
As the deferent duct straightens and leaves the corpus epididymis
(Fig. 26) it forms the vas deferens which eventually drains into the
prostatic urethra. This duct passes medially along the testis and
enters the abdominal cavity through the inguinal canal in association
with the nerves and blood vessels of the testis and epididymis.
The epithelium of the vas deferens is of the pseudostratified
ciliated type. A vell-developed lamina propria is present containing
many elastic fibers (Fig. 33). There is a layer of longitudinal smooth
muscle proximal to the lamina. A median, thick layer of mixed muscle
fibers (circular, oblique and some longitudinal) is present, surrounded
by a thinner layer of longitudinal muscle. This is almost identical to
the condition described in Myocastor (Stanley and Hillemann, 1960).
The urethra in the male, forming a common duct for the urine and
the products of the reproductive organs, is differentiated into prostatic,
membranous, and cavernous (penile) portions.
The prostatic urethra receives the neck of the bladder, the
ampullae, and the ducts of prostate and vesicular glands.(Figs. 25
and 34). The illustration of this region in Mus provided by Snell
(1941) would be indistinguishable from similar sections of Sigmodon.
The duct systems of the various glands entering this region have been
described under the descriptions of the respective glands.
Slightly posterior to the region of the prostatic urethra, the
nature of the urethra changes. Its body is surrounded by a mass of
striated muscle, the sphincter urethrae membranaceae. Between the
muscular wall and the lumen, a mass of loose, simple alveolar glandular
tissue forms the diffuse urethral glands (Fig. 17). A small amount of
smooth muscle is present in the lamina propria. The lining of this
region of the urethra is psuedostratified columnar epithelium.
After receiving the ducts of the bulbo-urethral glands and the
artery of the penis, the urethra turns ventrad and joins the crurae of
the penis. The urethra, now possessed of cavernous and erectile tissue
is known as the corpus cavernosus urethrae. The urethral diverticulae
described for Mus (Snell, 1941) are not found in Simodon.
The lining of the penile urethra is stratified columnar epithelium.
The penile urethra terminates in two lappet tips, associated with the
median process of the baculum as previously described.
General Circulation of the Reproductive Tract
The circulatory supply of the accessory glands, urethra and penis
is provided by branches of the hypogastric trunk.
The superior vesicular artery gives off branches to the following
glands: vesicular; ampullary; dorsal, ventral, and lateral prostates.
In addition, branches also supply the vas deferens and the bladder.
The inferior vesicular artery has a similar distribution, but is not as
All structures mentioned above are drained by veins which unite
to form the superior vesicular vein.
The preputial glands receive their blood supply from the superior
epigastric artery and the superior external pudendal artery. They are
drained by the superior external pudendal vein.
The bulbo-urethral glands are supplied by a small branch of the
artery of the urethral bulb, which in turn, is a branch of the artery
of the penis. The veins from these glands drain into the vein of the
urethral bulb which is continuous with the internal pudendal vein.
The internal pudendal artery gives off a branch, the artery of
the penis, which supplies the phallus and associated structures. The
artery of the penis gives off the following branches:
Artery of the urethral bulb this supplies the urethral bulb
and hence the corpus cavernosus urethrae and the bulbo-urethral
Urethral artery to the corpus cavernosus urethrae.
Deep artery of the penis to the M. corpus cavernosus penis and
the M. ischiocavernosus.
Dorsal artery of the penis to the surface of the corpus
cavernosus penis and surrounding connective tissues.
The corpus cavernosus penis of the body of the phallus is supplied
by the deep artery of the penis. The urethral artery and the artery of
the urethral bulb provide blood to the corpus cavernosus urethrae, which
in turn supplies blood to the associated circulatory structures of the
The blood of the penis is drained primarily by the dorsal vein of
the penis, which flows into the pudendal plexus at the base of the penis.
The blood from this plexus flows into the internal pudendal vein, to
the common iliac vein.
The testes are supplied by the spermatic arteries. These arise
from the dorsal aorta posterior to the renal arteries. The spermatic
arteries divide into external (to cremaster muscle) and internal branches.
The internal spermatic divides into the epididymal and testicular
arteries. The vas deferens, as previously noted, receives blood from
a branch of the superior vesicular artery.
The testes and epididymides are drained by the complex pampiniform
plexus, which eventually drains into the renal vein.
General Innervation of the Reproductive Tract
Sympathetic nervous system.-The prominent nerves of the male
reproductive tract are branches of the hypogastric plexus. This branches
to form two lateral plexuses on either side of the prostatic urethra.
The following smaller plexuses are branches of the pelvic plexuses:
Vesicular plexus innervates the vesicular glands, bladder,
ampullary glands, and vas deferens.
Prostatic plexus innervates the prostate glands and the
Further subdivisions of the pelvic plexuses join with branches of
the pudendal nerves (parasympathetic) and pass to the penis.
Parasympathetic nervous system.-The pudendal nerve receives
fibers from several sacral nerves. Branches extend to the scrotum, the
bulb of the urethra, M. bulbocavernosus, and penis. The final branch
of the pudendal nerve forms the dorsal nerve of the penis (receiving
sympathetic fibers from the prostatic plexus) and is distributed over
the surfaces of the corpus cavernosus penis and to all structures,
superficial and deep, of the glans penis (Fig. 35).
COMPARATIVE MORPHOLOGY OF THE MALE GENITALIA
OF MUROID RODENTS
There are available descriptions and illustrations of the complete
male genitalia for but three of 99 currently recognized genera of
cricetid rodents. These are Mesocricetus auratus (Cricetinae,
Cricetini), Neotoma sp. (Cricetinae, Hesperomyini), and Microtus
pennsylvanicus (Microtinae, Microtini). Such a dearth of information
is disappointing when one considers the vast literature that deals with
the baculum alone. Comparison of the reproductive structures of as
many genera as possible was made to determine the extent of variation
occurring in muroid rodents and the relation of these findings to the
present concept of taxonomy based upon other criteria (Table 1).
Several introductory comments are necessary in order that the
descriptions may be viewed in proper perspective. First, the genera
included are not truly of equal taxonomic rank. For example,
Peromyscus is composed of some 27 species, many rather diverse, which
may be subdivided upon further examination. On the other hand,
Ochrotomys is monotypic, until recently recognized as a subgenus of
Peromyscus. Onychomys and Baiomys include but two species each. As
it was obviously impossible to treat all the species of each genus
included, I have examined one species of each genus (Table 1), fully
realizing that further study on the larger genera may necessitate
additions to, or revisions of, certain of the conclusions drawn from
the present study. Secondly, variation within each form must be
considered. I have examined the genitalia of 50 specimens of Simodon
hispidus, and have found no variation in the basic number and morphology
of the accessory organs. This has also been the case with Ondatra.
Neofiber, Oryzomys and several other genera for which at least several
specimens were available. On this basis, therefore, I feel that the
data obtained for genera for which but a single adult specimen was
available are trustworthy. The number of specimens examined for each
genus is listed in Table 1.
Age and seasonal changes in forms where a seasonal reproductive
cycle occurs are other important sources of variation. Unless otherwise
noted, descriptions and illustrations are based upon adult specimens
with scrotal testes and in which the tubules of the cauda epididymis
were visible, two usual criteria for a fecund conditions in male rodents.
It is difficult, and probably fallacious, to give accurate measurements
of glands subject to such seasonal variability. Where given, the
measurements in this section are for the greatest length by the greatest
width of organs, unless otherwise specified. Care was taken that a
similar orientation of glands was used in all measurements.
Unless otherwise stated, the bulbo-urethral glands and their
duct systems are similar to those found in Sigmodon. These glands are
so alike in all genera examined that individual descriptions are not
With certain forms it is difficult to determine if a prostate
is bilobed, or whether two pairs are involved. I have attempted to follow
a conservative course, so that unless the glands were distinctly separate
with clearly unique duct systems, I have considered them to constitute
a single bilobed gland. The basic morphology of the gland (e.g., the
diameter of the tubules) was considered in making such decisions.
Of all the genera examined, only Rattus. Mus, and Mesocricetus
shoved variation in the nature of the accessory glandular structures.
In one specimen of Mesocricetus only one vesicular gland was present.
In a specimen of Rattus, the number of prostates was not equal on the
left and right sides of the urethra. These are also the only genera
examined that were from inbred strains of laboratory animals. Osteo-
logical variations in such strains of Rattus are well known, so there
is little reason that genital structures might not suffer similar
aberrations. Therefore, though I have figured what appeared to be
the "normal" condition, I have reservations about the descriptions of
these three genera.
Mesocricetus Nehring 1898
The phallus is of the complex type, and the surface of the glans
is spined. The accessory circulatory structures are well developed
(Fig. 6). The baculum has been described by Didier (1948) and Callery
(1951). Caller (1951) discusses osteogenesis of this bone and illus-
trates five stages in the development. The median distal process is
ventrally depressed, the urethra being completely blocked by this
process. The median process is usually the largest of the three digital
processes in animals possessing a trident-shaped baculum, but this is
not the case in Mesocricetus.
The accessory glands have been described in part by Ortiz (1947).
The ducts of the prostates are considered, but there is no mention of
the number of lobes, their extent, and other aspects of interest from
a taxonomic standpoint. The discussion of the embryology of the tract
and hormonally induced variation is noteworthy.
Preputial glands were not present in the specimens examined.
The vesicular glands are large and strongly recurved ventrally.
They are lumpy and irregular in shape, being quite bulbous on the
median surface near the ducts. A single duct from each gland enters
the anterior aspect of the prostatic urethra dorsal and lateral to the
vas deferens. The greater curve of the gland is heavily lobate, some
of the lobes being compoundly branched. Although a number of genera
exhibit some degree of location of the vesicular, the condition is most
pronounced in Mesocricetus.
The ampullary glands are large and lie at the point of contact
of the two vasa deferentia. The glandular tissue is not as tightly
bound by connective tissue as in Sigmodon. and consequently forms a
more amorphous mass.
Three pairs of prostates are present in Mesocricetus. A single,
elongate (12 x 5 mm) pair of ventral prostates lies lateral and caudal
to the bladder. The ducts enter the ventral wall of the prostatic
urethra lateral to the neck of the bladder. The paired dorsal prostates
are situated on the antero-dorsal wall of the prostatic urethra through
which the ducts pass. They encroach upon the dorsal margin of the
vesicular glands for a short distance. The colon passes between the
two lobes of this gland. The paired anterior prostates are located
within the lesser curves of the vesiculars and in contact with the
lateral borders of the latter glands. The ducts of the anterior pros-
tates, two on each side, enter the prostatic urethra adjacent to the
The testis, epididymis and vas deferens are not qualitatively
different from those of Sigmodon. The entire epididymis is invested
with yellow fat.
Oryzomys Baird 1857
The phallus is of the complex type; the surface of the glans is
spined. All accessory circulatory structures are well developed as
are the tendons and the M. suburethralis penis. The general appearance
is not unlike that of Sigmodon. The major differences lie in the
nature of the baculum, where the condyles are not as well developed as
in Sigmodon, and the cartilaginous distal processes are more attenuate.
One old individual examined had the distal processes partially
ossified. Hamilton (1946) first illustrated the baculum for 0. palustris;
Didier (1953) figures the shaft only. Burt (1960) figured this element
for 0. melanotis. Burt's illustrations are the best available. The
shaft is narrow at the distal end, flaring broadly at the base. The
dimensions of the processes vary in the different species.
A single pair of preputial glands is present. These are large
and flaring at the proximal end. The main bodies of the glands lie on
the sides of the glans penis. Each is drained by a single duct that
opens at the distal, lateral edge of the prepuce.
The vesicular glands are large and recurved in Oryzomys. with
the tips in contact with the base of the gland. They are very lobulated.
Laterally, the gland appears to be a solid mass but its recurved nature
is readily apparent upon dissection. The single duct of each gland
enters the prostatic urethra immediately lateral to the vas deferens.
The ampullary glands are compact, each being adjacent to the
respective vas deferens. Each gland is drained by numerous ducts,
opening into the vas deferens just distal to the ampulla.
Four pairs of prostates are present. The two pairs of ventral
prostates consist of a median pair (10 x 3 mm) lying caudad of the
bladder, and a more lateral, bulbous pair (6 x 5 mm) situated on either
side of the median pair. The ducts of the former enter the urethra
posterior to the neck of the bladder, while those of the latter enter
slightly more laterally. The median ventral prostates are white in
formalin or FAA, while the lateral ones are red in the same. A single
pair of elongate dorsal prostates (8 x 4 mm) is situated on the dorsal
surface of the urethra, encroaching anteriorly onto the dorsal border
of the vesiculars. The colon passes medially between the members of
this pair. The ducts enter the antero-dorsal wall of the prostatic
urethra. The anterior prostates are filiform, fanlike glands, lying
upon the median surfaces of the vesiculars (Fig. 37, B). They are
drained by two ducts on each side, the ducts entering the urethra
lateral to those of the vesiculars.
Oecomys Thomas 1906
I was able to obtain only the proximal portion of the tract
so I cannot comment upon the phallus, preputial or bulbo-urethral
glands. Hooper (1959) mentions that the phallus is complex in nature,
but does not elaborate.
The vesicular glands are lumpy and recurved. The median border,
as well as the greater curve, are both irregular in shape. The recurved
tip is strongly notched. Each gland is drained by a single duct
entering the urethra dorsal and lateral to the ampulla.
The ampullary glands lie upon the corresponding vas deferens to
which numerous ducts pass in the region of, and immediately preceding,
Four pairs of prostates are recognizable in this genus. An
elongate (9 x 3 mm), median pair of ventral prostates lies directly
caudad of the bladder, and a second, more bulbous (3 x 3 mm), lateral
pair is located on the sides of the prostatic urethra. The median
pair is white in formalin, the lateral pair a reddish purple color.
The ducts of the median pair enter the prostatic urethra posterior
to the neck of the bladder, the other pair enter the urethra through
the lateral walls. The dorsal prostates are represented by a pair
of compact glands at the anterior end of the urethra. The ducts enter
the urethra through the antero-dorsal wall. The anterior prostates
are a loose, filiform set of glands lying in the lesser curve of the
vesiculars. These are drained by a pair of ducts on each side that
enter the urethra lateral to the ducts of the vesiculars.
Nectomys Peters 1861
The phallus of this genus has not previously been described
although Hooper (1960) mentioned that it was of the complex type. The
glans is spined, and its internal architecture is basically similar to
Sigmodon. The distal processes of the baculum and associated circulatory
structures are large and bulbous. The lateral processes are larger than
the median, similar to those of Mesocricetus (Fig. 6). The median
process is smaller, ventrally depressed, and blocks the urethra. The
M. suburethralis penis and the dorsal tendon of the penis are well
developed. Burt (1960) illustrated this element of N. squamines in
part, the distal processes being omitted. The shaft is broad, and
slightly curved. Its base has two well-developed condyles and as
mentioned above, the distal processes are well developed. In the
specimen examined, the latter were not ossified.
A single pair of preputial glands is present, situated on the
lateral borders of the glans penis. These are long and narrow,
measuring 14 x 4 mm each. Each is drained by a single duct.
A pair of bulbo-urethral glands is situated in approximately the
same position as described for Sigmodon, though the median border of
each gland is in contact with the rectum in Nectomys. In the other
genera examined the gland is not located so deep. Each gland is
drained by a single duct that enters the dorso-lateral wall of the
membranous urethra anterior to the origin of the phallus.
The vesiculars are large and ventrally recurved. The gross
appearance is one of extreme lumpiness. Members of the pair are
asymmetrical with the median borders being more irregular. The greater
curve is grossly lobate, as opposed to the fine compound lobations
present in Mesocricetus. Each gland possesses a single duct that opens
into the cephalic end of the prostatic urethra adjacent to the ampulla.
The ampullary glands are compact and lie about the vas deferens.
They encompass an area about 4 x 3 x 2 mm. Numerous ducts drain each
Five pairs of prostates are present in Nectomys. There are two
elongate pairs of ventral prostates. The median pair lies directly
caudad of the bladder, the ducts entering the prostatic urethra through
the ventral wall posterior to the neck of the bladder. The lateral
pair is of the same size (9 x 3 mm), but is drained by ducts entering
the urethra lateral to the neck of the bladder. The median pair
remains white in FAA the lateral pair assumes a red color. A typical
pair of dorsal prostates lies at the anterior portion of the tract on
the dorsal surface of the prostatic urethra. The ducts from these
glands enter the urethra through the antero-dorsal wall. The bodies
of these glands encroach upon the dorsal surface of the vesiculars.
The colon passes between the two lobes of the glands. A smaller,
more lateral pair of glands is situated on the sides of the prostatic
urethra below the dorsal prostates. Their ducts seem, however, to open
into the prostatic urethra next to the ducts of the dorsal prostates.
Because of their general morphology (the tubules appear identical to
those of the dorsal prostates) and the proximity of the ducts, I
consider these glands to be a second dorsal set, though more laterally
located. Both sets of dorsal prostates are white in FAA. The filiform
anterior prostate glands lie in the lesser curve of the vesiculars.
Because of the great asymmetry of the vesiculars these prostates lie
on both lateral and median aspects of the vesiculars. Each gland is
drained by a pair of ducts entering the prostatic urethra close to the
duct of the vesicular.
Akodon Meyen 1833
Hooper (1959) mentioned that the glans penis was of the complex
type, but did not elaborate further. The accessory circulatory
structures are small (Fig. 9). The penile blood sacs are the smallest
in any complex phallus I have examined. The distal architecture is
small (reduced?), and the accessory circulatory structures are corres-
pondingly reduced. The shaft of the baculum is laterally compressed
and almost bladelike (Fig. 11). The base is somewhat expanded. The
three digital processes are small, the median most ventral in position
and apparently partially blocking the urethra. The median process is
the largest, and all three processes are cartilaginous.
There are two pairs of preputials as in Sigmodon, a small
ventral pair (2 x 1 mm) and a larger lateral pair (5 x 3 mm). Each
gland is drained by a separate duct. The lateral pair has a flaring
proximal end and a ventral concavity in which lies the small pair.
The bulbo-urethral glands are in the same position as in Sipmodon,
each gland being drained by a single duct entering the dorso-lateral
wall of the membranous urethra anterior to the origin of the phallus.
The vesicular glands are small and recurved and slightly scalloped
along the greater curve. Each is drained by a single duct, which
enters the anterior wall of the prostatic urethra adjacent to the vas
The paired ampullary glands do not differ in any fundamental way
from those of Sigmodon.
Three pairs of prostate glands are recognized in this genus. The
single pair of ventral prostate glands is median in position, the ducts
entering the prostatic urethra through the ventral wall posterior to
the bladder. The dorsal prostates lie somewhat anterior to the urethra,
encroaching upon the dorsal border of the vesiculars. Their ducts
enter the antero-dorsal wall of the urethra. The colon passes medially
between the two lobes. A single pair of anterior prostate glands is
present, although each has a bilobed appearance. One lobe runs craniad
along the lateral border of the corresponding vesicular. The second lobe
is a small rosette of prostatic tissue situated on the ventral border
of the vesicular prior to the entrance of that gland into the prostatic
urethra. The ducts of the two lobes of each side of the anterior
prostate glands are indistinguishable, however. All prostates were
white in preservative.
The specimen examined had small blotches of pigment in the tunica
which gave the testis a "polka-dot" appearance. Whether this pigment
pattern is characteristic of Akodon or simply an artifact is not
certain. The testis of the specimen studied was also slightly irregular
in outline, being depressed in several places by the epididymis.
Phyllotis Waterhouse 1873
The phallus of Phyllotis is complex and the surface of the glans
is spined. Pearson (1958) illustrated the partially dissected glans
and the baculum of three species.
The preputial glands were not present on the specimen I examined.
However I had only a carcass remaining from a skin preparation, and it
is not certain that these glands had not been lost.
The paired bulbo-urethral glands lie in the same position described
for Signodon. The gland body is not pearshaped as in Sigfodon but is
concave on one side forming a "saucer-like" structure.
The vesiculars are large and ventrally recurved toward the rear.
The outer margin is slightly lumpy, while the lesser curve is smooth.
The duct arrangement is similar to that of Signodon.
The ampullary glands are well developed, the greatest mass of
the gland being concentrated on the dorsal aspect of the vas deferens.
Each gland is slightly bilobed.
There are four pairs of prostate glands. A bulbous, median pair
of ventral prostates lies caudad of the bladder. The ducts enter the
urethra through the dorsal wall posterior to the neck of the bladder,
the ducts following the neck into the prostatic urethra. A smaller,
lateral pair of ventral prostates lies along the neck of the bladder,
at which point the ducts enter the urethra. The dorsal prostates lie
on the dorsal wall of the prostatic urethra and partially encircle the
colon. The ducts enter the antero-dorsal wall of the prostatic urethra.
The paired anterior prostates lie in the lesser curve of the vesiculars.
Each enters the prostatic urethra at its anterior border by two ducts.
All prostates are white in preservative.
The testis is indented by the cauda epididymis. The corpus
epididymis is very narrow and almost indistinguishable. The vas
deferens exhibits no unique features.
Nyctomys De Saussure 1860
The glans is covered by spines, which are small distally but
become larger at the proximal end. The spines at the proximal portion
are the largest in any genus in this study. The structure of the glans
penis is complex, although a reduction in the complexity is evident.
Thus the distal processes are small and almost lacking in cartilaginous
tissue. The circulatory tissues of the distal portion comprise most
of the mass. The penile blood sacs are not proportionally reduced.
Burt (1960) illustrated part of the baculum, but did not include the
digital processes. The base of the specimen I have examined is more
expanded than in Burt's illustration, though this may be due to the age
of the specimen.
No preputials were found in the specimen examined.
The paired bulbo-urethral glands are situated in the same position
as described for Sigmodon, although they are more ovate than in the
latter. A single duct drains each gland and enters the membranous
urethra anterior to the origin of the phallus.
No vesiculars were present in the specimen examined.
The ampullary glands are large, 5 x 5 mm, and are loosely filiform
in nature in contrast to the more compact type exhibited by Sigmodon and
some other genera. Each gland empties into its corresponding vas defer-
ens by numerous ducts prior to the formation of the ampulla.
Only a single pair of prostates are present in this genus, forming
the most anterior extension of the tract, and apparently represent the
anterior prostate series. On each side there are four units, each
drained by a single duct entering the antero-dorsal wall of the prostatic
urethra lateral and dorsal to the ampulla. These units are long and
thin (15 x 3 mm) and are composed of two to eight branching tubules.
The only other genus in which the prostates are similar to Nyctomys
to Neotoma, though Nyctomys lacks the ventral prostates which are
present in Neotoma. The phallus of Nyctomys is complex however,
while that of Neotoma is simple.
The testes are stubby and blunted at the ends in the specimen
available. There is little fat deposition about the epididymis. The
cauda epididymis does not cap the posterior pole of the testis as is
the usual case, but instead lies to one side and in contact with
Peromyscus Gloger 1841
The phallus of Peromyscus has been the subject of a comprehensive
study by Hooper (1958) in which many species are described and figured.
The phallus is of a simple architecture with none of the additional
circulatory structures associated with the complex type of phallus.
The glans of some species bear spines. The specific variations in the
bacula of numerous species of Peromyscus have been described by a
number of different workers: Sprague (1939), Blair (1942), Clark (1953),
Hooper (1958), Tamsitt (1958), and Burt (1960). The baculum of this
genus is a simple rod-shaped structure, dorso-ventrally flattened with
a laterally expanded basal portion.
Preputial glands were not present in the specimen of this genus
The vesicular glands are large and recurved, measuring about 50 mm
along the greater curve. The margin of the greater curve is very
prominently notched, while the lesser curve is smooth. A single duct
drains each gland.
The ampullary glands lie along the vas deferens as they join
craniad to the neck of the bladder. The ventral aspect of the vas
deferens is not covered by ampullary tissue. The total area of the
glands is about 4 x 4 x 3 mm, a considerable portion of which lies on
the cranial border of the vas deferens.
Four pairs of prostate glands are distinguishable in the specimens.
A medial pair of ventral prostates is located directly caudad of the
bladder (8 x 5 mm). The second, lateral to the bladder, (10 x 7 mm) has
a concave ventral surface through which passes the vas deferens. The
ducts of both pairs of ventral prostates enter the ventral side of the
urethra. The dorsal prostate glands lie on the most antero-dorsal
portion of the prostatic region, abutting against the vesiculars. The
descending colon passes between the two lobes. The ducts enter the
dorsal aspect of the urethra. The anterior prostate glands are long
attenuate glands lying in the lesser curve of the vesiculars. They
measure 12 x 3 mm. Each gland is drained by two ducts that enter the
urethra lateral to, and in contact with, the vesicular ducts. All
prostates are while in preservative.
Both the caput and cauda epididymis are invested with yellowish-
green fat, and the corpus epididymis is very thin. The cauda epididymis
is attached to the testis by a mesentery but does not cap the posterior
pole of the testis as in Sigmodon.
Reithrodontomys Giglioli 1873
The phallus of this genus has been figured by Hooper (1959).
The baculum Is a simple rod, expanded at the base, and has been
described for several species by Benson (1939), Blair (1942), Hooper
(1959), and Burt (1960).
Preputial glands could not be observed in the specimens
The horns of the recurved vesiculars are quite lumpy, curving
ventrally then folding back upon themselves. Along the greater curve
they may measure 15 mm. A single horn of the vesicular may contain
as many as eight lumpy protuberances. The ducts lie lateral and dorsal
to the vas deferens.
Each member of the paired ampullary glands lies in association with
the corresponding vas deferens to which it is adnate. They are located
craniad to the bladder in the region in which the vasa deferentia join.
The ampullary glands are compact and each covers the vas deferens with
which it is associated on all but the ventral surface.
There are four distinct pairs of prostates developed in this genus.
A bulbous (4 x 3 mm), median pair of ventral prostate glands lies
caudad to the bladder. This pair is white mhen fixed in FAA. Dorsal
and lateral to the median pair is a smaller (1 x 1 mm) set, which is
also white in FAA. The ducts of both of these pairs of glands enter
the ventral and lateral walls of the urethra. The dorsal prostates lie
on the dorsal and most anterior portion of prostatic urethra and are
separated by the colon. The gland, which is pink in FAA, drains into
the antero-dorsal portion of the urethra. One lobe measures 3 x 2 mm.
The anterior prostates are elongate (6 x 3 mm), lying in the lesser
curve of the vesicular, and draining, by two ducts on each side,
parallel to the ductus deferens and openings of the vesicular ducts.
The caput and cauda epididymis are invested with yellow fat.
The cauda epididymis is attached to, but does not cap, the posterior
pole of the testis.
Neotoma Say and Ord 1825
The architecture of the phallus is simple; the glans is spined.
Hooper (1960) reviewed the glans penis in neotomine rodents and figured
this structure for N. floridana. The baculum is not constant within
the group, ranging from an elongate, gracefully curved structure as in
N. lepida to a small, almost triangular one in N. fuscipes. The
diversely shaped bone of this group has been described for numerous
species (Howell, 1926; Burt and Barkalov, 1942; Hoffmeister and de la
Torre, 1959; Hooper, 1960; and Burt, 1960).
The accessory glands have been figured by Howell (1926). Preputial
glands are not present in the members of this genus that I have examined.
Howell (1926) commented on a small sac-like structure in _. intermedia
that may have represented a vestigial preputial.
The bulbo-urethral glands are more flattened than is the case
for the other genera examined.
Vesicular glands are not present in the specimens of this
genus I have examined.
The ampullary glands are quite large and diffuse. Combined they
measure 15 x 10 mm, extending some 5 mm craniad of the anterior
extension of the vas deferens. There are numerous ducts from each lobe
of this gland entering the corresponding vas deferens for the final
5 7 mm of this organ.
There are two pairs of prostate glands. The ventral prostate
glands are long and narrow and the tubules are filiform. Each lobe
measures over 12 mm long, the widest is 6 mm. The ducts enter the
dorsal aspect of the urethra lateral to the neck of the bladder. A
very large and diffuse pair of anterior prostate glands is located
in the position usually occupied by the vesicular. The tubules are
heavily branched, assuming a fan-like appearance. The glands are
some 25 30 mm wide in greatest expanse. The tubules are widest
near the bladder, becoming more narrow as they continue to branch.
There are two ducts on each side entering the prostatic urethra lateral
to the ductus deferens.
Howell (1926) described and figured the accessory glands of
leotoma. The glands that are designated as "coagulating" by him
apparently are the ampullary glands, while those denoted as "vesicular"
represent one of the prostate series. According to my interpretation,
vesiculars are not present in this genus. The branched, filiform
nature of the glands in question is characteristic of the prostate
It is not clear which of the prostate glands are represented by
the "vesiculars" of Howell. There is no typical dorsal prostate in
1leotoma. Thus, these fan-shaped glands could be either highly modified
anterior or dorsal prostates. I have referred them to the anterior
prostates for the following reasons:
1) The tubules of the dorsal prostates are generally of a smaller diameter
than the glands in question.
2) The ducts (two on each side) enter the prostatic urethra lateral to
the ductus deferens, as is the usual condition for the anterior prostates.
3) The fan-like nature of these glands can be more easily derived from
the usual condition of the anterior, rather than dorsal, prostates.
Most forms that possess a large anterior prostate also have a large,
recurved vesicular. It is within the lesser curve of these vesicular
glands that the anterior prostate usually lies. If the vesicular
glands drop out, the anterior prostate, with its filiform nature, would
probably expand, producing an effect similar to the condition found in
The testes never become completely scrotal (Howell, 1926). The
cauda and caput epididymis are invested with yellow fat. The cauda
epididymis does not cap the posterior pole of the testis. The corpus
epididymis is very broad. The vas deferens is not convoluted at any
point. There is a gradual swelling of the vas deferens for about the
distal half of its length.
Tylomys Peters 1866
Hooper (1960) described the glans penis of this genus. The
structure of the phallus is of the simple type, and the surface is
spined. The baculum in the two species described, T. fulviventer and
T. nudicaudus (Hooper, 1960; Burt, 1960), is a simple, rod-shaped
bone, expanded at the base and capped at the distal end by a carti-
I cannot confirm the presence or absence of the preputial glands
in this genus as the specimen available to me was lacking the distal
The paired bulbo-urethral glands are similar to those in the
other forms examined.
The condition of the ampullary glands are the most distinctive
feature of the male tract of this genus. Each ampullary gland is
associated with a corresponding ampulla. The ducts, several from each
gland, enter the ampulla (Fig. 46, B) lateral to the entrance of the
vas deferens. The ampulla is the largest and most well developed of
any cricetid examined. It is bulbous, with the ampullary gland attached
laterally appearing as a diverticulum from the ampulla. The gland is
about 3 mm long, and folded upon itself. This condition is unlike
that found in any other cricetid genus examined in this study.
The vesicular glands are lumpy and irregular in shape. They are
not truly recurved in the manner of Sig~odon, but instead have the
anterior ends twisted and lying upon the main body of the gland.
There are only two pairs of prostate glands in this genus. A
small (5 x 3 mm), median, compact pair of ventral prostate glands is
located caudad of the bladder, the ducts entering the ventral wall of
the urethra just posterior to the neck of the bladder. A second, more
diffuse pair of prostate glands is situated at the anterior end of the
prostatic urethra on the dorsal border. These ducts enter the urethra
at this point. These glands are filiform as is often the case with the
anterior prostates, but occupy the position of the dorsal prostates of
the other genera. About 6 8 unconvoluted tubules constitute each
part of the gland. These are more easily seen and dissected in a
specimen that has not reached full sexual development, than in an adult,
as in the latter, they are too distended and associated with much fatty
The testes are large, 26 x 15 mm, in the slightly desiccated
specimen available, and the cauda epididymis lies to one side of the
testis (Fig. 46, C) rather than capping the posterior pole. The cauda
and caput epididymis are lumpy and irregular on their surfaces, the
latter being invested with yellowish fat.
Balomys True 1894
Hooper (1959) figured the phallus of B. musculus. The plan of
the glans is simple, and the surface is spined. The baculum is short,
irregularly expanded at the base and faintly forked at the distal end.
This element has been figured for B. musculus by Hooper (1959) and
Burt (1960). The latter illustrates two specimens that are quite unlike.
Hooper's illustration is different still. The baculum is either extremely
variable in this genus or some other explanation must be afforded for
the discrepancies between the published accounts of this element.
Blair (1942) described this element in B. taylori.
A single pair of preputials is located on the sides of the
glans. They are small (5 x 2 mm), expanded proximally, and drained by
a single duct on each side.
The vesicular glands of Baiomys are quite simple and small
(8 x 1.5 mm). They are straight for most of their length, and recurved
only at the tip.
The ampullary glands are the most reduced of any genus studied.
They consist of four to six simple tubular glands draining directly into
the corresponding vas deferens. The individual tubular elements are
not connected. They are very small, being less than 1 mm in length.
Paired ventral, dorsal, and anterior prostates are present. The
former are located posterior to the bladder and measure 4 x 3 mm.
Their ducts enter the ventral wall of the prostatic urethra caudad
of the neck of the bladder. The dorsal prostate glands lie on the
anterior cephalic border of the prostatic urethra, the descending colon
passing between the two lobes of the gland. The ducts enter the dorsal
vall of the prostatic urethra. The anterior prostates lie at the cephalic
end of the urethra and extend along the border of the vesiculars but are
not adnate thereupon. They are about 4 mm long, and filiform in appear-
ance. All prostates are white in FAA.
The testis appears almost spherical (4 x 3 mm), and the cauda
epididymis which caps the posterior pole is large and invested with
Ochrotomys Osgood 1909
The glans penis (Fig. 8) is of the simple type (Hooper, 1958).
The glans is spined, these appearing somewhat larger than Hooper's
description of them as "minute." The distinctiveness of the glans
penis-together with dental and certain other characteristics prompted
the recognition of the generic status of Ochrotomys whereas it had
previously been considered as a subgenus of Peromyscus (Hooper, 1958).
The baculum is a small, blunt rod with a laterally expanded basal
portion. The bone has been figured by Hooper (1958) and Burt (1960).
A pair of preputial glands is present, lying on the sides of the
glans penis. They are small, about 4 x 1 mm, a single duct draining
each of these glands.
Ochrotomys is characterized by small vesicular glands which
appear as simple tubular structures. They are about 10 x 2 mm and
slightly recurred near the tip. The recurving appears to follow no
No ampullary glands could be demonstrated in dissection.
Four pairs of prostate glands can be recognized. A single median
pair of ventral prostate glands lies posterior to the bladder. These
are small and bulbous. A second pair, loosely divided into two lobes
on each side is situated lateral to the neck of the bladder. The
ducts of the two sets enter the urethra on the dorsal aspect, posterior
and lateral to the bladder respectively. Both sets are white in FAA.
The dorsal prostate glands are small and lie at the anterior end of the
urethra on the dorsal side. The pair cradles the descending colon. The
ducts enter the prostatic urethra dorsally.
The anterior prostate glands lie at the base of the vesiculars and
enter the prostatic urethra by what appears to be a single duct from
each side, situated next to the vas deferens. Since the vesiculars are
reduced in size these glands have a more compact nature than in those
forms which have a large vesicular upon which to spread. From the median
pair of ventral prostates to the dorsal prostate there is but a mass of
prostatic tissue broken into the units previously described. Of all
forms described, this one has the most poorly defined sets of prostates.
The anterior prostate is not typical, but the ducts are in the proper
position. There appears to be but one duct from each member of the pair,
whereas most other genera examined have two.
The testis is relatively large (11 x 6 mm). The cauda epididymis
caps the posterior pole of the testis. The epididymis is invested with
yellowish-green fat. The vas deferens is slightly convoluted throughout
Onychomys Baird 1857
The phallus which has been figured by Hooper (1959), is of the
simple type and the glans is spined. Hooper (1959) illustrated 0.
torridus, but commented that leucogaster was similar, differing only
in proportions. The bacula of both species have been described and
figured by Burt (1960), and Hooper (1959) illustrated 0. torridus.
"It is a short bone with a round knob for a base, and with a laterally
flattened shaft" Burt (1960).
A pair of preputial glands lie on the lateral sides of the glans
penis, the ducts opening near the distal end of the prepuce. These glands
are long and narrow (8 x 2 mm).
The bulbo-urethral glands are large (4 x 3 mm) in this genus but
otherwise similar to those of Sigmodon.
The male genital tract of Onychomys is distinctive in that both
vesicular and ampullary glands are absent. Neither vesicular nor
ampullary glands were seen in the specimen examined.
The genus possesses two pairs of prostate glands. A single
median pair of bulbous (4 x 3 mm) ventral prostates are present. These
are white when preserved in FAA. The ducts enter the urethra on the
ventral surface lateral to the neck of the bladder. A median inflated
(4 x 3 mm) pair of dorsal prostate glands is located at the anterior
portion of the prostatic urethra in the position usually occupied by the
absent vesicular. The lobes are not separated by the colony The ducts
enter the urethra dorsal to the vas deferens.
The testis appears long and narrow (11 x 5 mm). The cauda epididymis
caps the posterior pole of the testis, and tapers to form a cone with a
rounded tip. The vas deferens is heavily convoluted throughout its
length, a condition not observed in any other genus examined.
Synaptomys Baird 1857
The phallus is of a complex nature, the surface of the glans is
spined. The structure of the glans penis of this or any other microtine
does not seem to differ from that of Sifmodon. All accessory circu-
latory structures recognized in Sigmodon are present. The baculum of
this genus was first figured by Hibbard and Rinker (1912) for S. cooper.
Hamilton (1946) demonstrated the trifurcate nature of this element,
not noted in the previous work. Dearden (1958), Burt (1960) and Ander-
son (1960) have all figured this element and discussed its structure.
A single pair of preputials is present and are proportionally the
largest in any genus examined, spreading out upon the M. rectus abdominis
from their origin in the tissues of the prepuce. They were 15 x 10 mm
in an adult specimen and are drained by a single duct on either side of
the glans penis. The broad flaring preputials are a particularly
striking feature of the male tract of Synaptomys, even for the microtines,
where preputials average proportionally larger than those of any other
The vesicular glands are well developed and of a simple tubular
nature, They are irregular in diameter, and lumpy at certain points,
though the general appearance is not one of extreme irregularity. They
are convoluted at the ends, though the convolutions do not seem to
follow any particular pattern. One measures 15 x 3 mm when straightened.
The vesiculars empty into the prostatic urethra adjacent to the vas
deferens by means of a single duct from each gland.
The ampullary glands are small, tightly knit glands lying adnate
to their corresponding vas deferens into which they are drained by
numerous ducts. Each ampullary gland appears to contain about eight
compound bulbous units, tightly bound about the vas deferens.
Four pairs of prostate glands are recognized. One major pair of
ventral prostates lies slightly posterior and lateral to the neck of
the bladder. These are elongate (8 x 4 mm). A second smaller pair of
ventral prostates is present at the base of the preceding, though the
ducts are located slightly closer to the lateral border of the neck
of the bladder. These two sets of glands are not as clearly distinct
as those of some other genera. Both are white in FAA. The ducts of
both sets pass through the ventral wall of the prostatic urethra, poster-
ior and lateral to the neck of the bladder. The dorsal prostates are
large (8 x 3 mm), situated on the antero-dorsal border of the prostatic
urethra, and extend somewhat cephalic of this structure. The colon
passes between the lobes of this paired gland. The ducts enter the
antero-dorsal wall of the prostatic urethra. The anterior prostates are
attenuate glands lying adnate to the dorsal border of the vesiculars for
about 8 mm. They do not closely resemble the anterior prostates of some
of the cricetids such as Signodon or some other microtines such as
Ondatra. The ducts, two on each side, enter the anterior wall of the
prostatic urethra ventral and lateral to the ducts of the vesicular
The testis is rather small (7 x 5 mm), capped at the posterior
pole by the cauda epididymis. The whole of the epididymis, especially
the caput, is invested with greenish-yellow fat.
Ondatra Link 1795
The glans penis is complex and the surface spined. The arrange-
ment of accessory circulatory structures is similar to that described
for Signodon. The penile blood sacs are well developed, as are the
tendons and muscular components of the phallus. As the largest of
cricetid rodents, the parts of the phallus are considerably larger, and
more developed than in smaller forms, though not disproportionately.
The baculum has been figured by Hamilton (1946), Didier (1954), and
Burt (1960). Anderson (1960) described, but did not figure this bone:
he commented that Hamilton's interpretation of dorsal and ventral was
not correct. The element is the largest trident-shaped bone seen. All
digital processes are somewhat spatulate, with an uneven border. The
median process is deflected ventrally, and a small keel is present
separating the lappet tips of the urethra in the flaccid condition.
The large, paired preputial glands spread across the M. rectus
abdominis to a considerable extent. The largest measured wore almost
50 mm long x 10 mm wide. Each gland is drained by single duct lying on
the dorsal aspect of the glans penis.
The large, heavily convoluted, tubular vesicular glands are very
irregular. There is no apparent pattern to the twisting and turning,
other than accomodation to the other organs of the abdominal cavity.
There is moderate branching and location, but the general simple tubular
nature of the glands is not obscured. Straightened, a single vesicular
may measure as much as 75 mm. For comparison a human vesicular measures
about 100 mm (Weyrasch, 1959). Each gland drains through the anterior
wall of the prostatic urethra by means of a single duct that lies dorsal
and lateral to the ampulla.
The ampullary glands are attached to the vas deferens, draining
into it by means of numerous ducts at the point at which the vas deferens
enters the ampulla. The total area of these glands is approximately
15 x 10 x 5 mm. Three ventral and four dorsal lobes can be distinguished
in each gland.
There are four pairs of prostates. An elongate (20 x 10 mm)
pair of median ventral prostate glands lies posterior to the bladder.
The ducts enter the ventral wall of the prostatic urethra directly
caudad of the neck of the bladder. These glands are white in FAA. A
smaller (5 x 4 mm), more bulbous and lateral pair of ventral prostate
glands that is purple in FAA lies at the base of the larger glands.
The ducts enter the urethra near the neck of the bladder. The dorsal
prostates are large (15 x 10 mm) glands lying on each side of the
colon and on the antero-dorsal border of the prostatic urethra. A
large, 25 x 8 mm, pair of anterior prostate glands lies upon the
ventro-lateral surfaces of the vesiculars. Each opens into the prostatic
urethra by four ducts that lie next to the vesicular ducts.
The testis is large (25 x 12 mm), the cauda epididymis lying at
the posterior pole, but not "capping" it. The epididymis is invested
with yellowish fat. The vas deferens is unconvoluted.
Neofiber True 1884
The phallus is of the complex type, the surface of the glans
penis being spined. The internal structure of the glans is very
similar to that of Sigmodon (Fig. 8). The baculun was described by
Hamilton (1946) and Anderson (1960). The median process of the baculum
is not ossified in young specimens, but in some older individuals this
ossification does occur. Whether the ossification center occurs in all
individuals is, however, questionable.
A large pair of preputial glands is present. They cover part of
the M. rectus abdominis. They are proximally expanded, measuring 20 x
10 mm. Each gland is drained by a single duct that lies on the lateral
aspect of the glans penis.
The vesicular glands are large, convoluted tubular glands that
measure as much as 50 mm when straightened. They are fairly uniform
(about 5 7 mm) in thickness. The convolutions are either medial or
lateral, and appear to have no constant pattern. Each vesicular gland
is drained by a single duct which opens into the prostatic urethra
adjacent to the vas deferens.
The ampullary glands partially surround the vas deferens, anterior
to the neck of the bladder. Each gland is associated with its corres-
ponding vas deferens, into which enter numerous ducts. These glands,
like the corresponding ones in Ondatra are divided into several lobes
which are tightly bound together.
Three sets of prostate glands can be recognized. A very large
pair of ventral prostates is located directly posterior to the bladder.
These glands measure 18 z 11 mm in length and width in one adult
individual. This pair of glands may be teased apart into two lobes on
either side. However, since there seems to be but one duct system
entering the prostatic urethra, posterior and slightly lateral to the
neck of the bladder, it is assumed that only one pair of glands is
represented. A single pair of dorsal prostates are present, located on
the antero-dorsal wall of the urethra, the colon passing between the
two lobes. A single gland measured 11 x 5 mm in length and width. The
ducts enter the urethra through the antero-dorsal wall. The paired
anterior prostate glands lie on either the lateral, dorsal, or ventral
borders of the vesiculars, and are drained by four ducts on each side.
The ducts enter the prostatic region lateral to the ducts of the
vesicular glands. In one specimen, I had difficulty separating these
from the dorsal prostates to which they were bound with connective
tissue. This emphasizes the importance of examination of duct systems
rather than masses of glandular tissue in distinguishing between these
various prostatic units.
Microtus Schrank 1798
The structure of the phallus is of the complex type and the
surface of the glans is spined. I have not examined the architecture of
the glans in detail, but it does not seem to differ from that of
Neofiber (Fig. 8) in any appreciable manner. The baculum of the follow-
ing species of Microtus have been described or figured: M. agrestris
(Ognev, 1950; Anderson, 1960), M. arvalis (Ognev, 1950; Didier, 1954;
Anderson, 1960), M. californicus (Anderson, 1960; Burt, 1960), M.
chrotorrhinus (Hamilton, 1946), M. fortis (Ognev, 1950; Anderson, 1960),
}. gregalis (Ognev, 1950; Anderson, 1960), M_. guatemalensis (Anderson,
1960), M. guentheri (Anderson, 1960), M. longicaudus (Anderson, 1960;
Burt, 1960), M. mexicanus (Anderson, 1960; Burt, 1960), M4. miurus
(Anderson, 1960), M. montanus (Anderson, 1960), M4. nivalis (Didier,
1954; Anderson, 1960), Mf. oeconomus (Ognev, 1950; Anderson, 1960; Burt,
1960), 14. oregoni (Anderson, 1960), 1M. orcadensis (Anderson, 1960),
E_. pennsylvanicus (Hamilton, 1946; Dearden, 1958; Anderson, 1960;
Burt, 1960), M. richardsoni (Anderson, 1960), M. socialis (Argyropulo,
1933; Didier, 1954), and M. townsendii (Anderson, 1960). Hamilton
(1946) vas the first to note the presence of the proximal flaring of
the median process, and the keel ("spur") of that element.
Sough the tract was described by Hamilton (1946), my interpretation
of some of the structures does not coincide with his.
A single pair of preputial glands is present, lying on the lateral
sides of the glans penis. A single duct drains each gland. As
Hamilton (1941) states, these are small as compared to those of
Synaptomys and Clethrionomys.
The recurved vesicular glands are not unlike those of Clethrionomys
or Sigmodon, though they are smaller. The greater curvature is lumpy,
the lesser more smooth. Each gland drains into the prostatic urethra
by means of a single duct that courses dorsal and lateral to the vas
The paired ampullary glands lie upon the vas deferens at the
point at which the latter converge, bend, and enter the urethra. Each
gland is associated with its corresponding vas deferens. The glands here
considered to represent the ampullary glands are evidently the "ventral
lobes" of the prostate of Hamilton (1941).
Four pairs of prostate glands are recognized in Microtus.
A median, somewhat elongate (6 x 3 mm) pair of ventral prostate glands
lies posterior to the bladder. A second, more bulbous (4 x 3 mm)
pair of ventral prostates is situated more laterally. The ducts of the
median pair enter the prostatic urethra posterior to the neck of the.
bladder, those of the lateral pair enter the urethra near the sides of
this structure. Hamilton (1941) did not mention these glands, though
they appear in his illustration. The paired dorsal prostate glands
lie along the anterior cranial border of the prostatic urethra. The
anterior margins of these glands sometimes encroach upon the dorsal
border of the vesicular gland on the corresponding side. The ducts
enter the urethra through the antero-dorsal wall, under the main body
of the gland. Hamilton (1941) did not discuss these glands, though
they are visible in his dorsal view of the tract. The paired anterior
prostate glands lie in the lesser curves of the vesicular glands,
along the lateral borders. They are drained by two ducts on each side
which enter the urethra adjacent to the ducts of the vesicular glands.
These are the "ventro-lateral lobes of the prostate" of Hamilton
The testis, epididymis, and vas deferens are usual in all
regards, and are adequately described by Hamilton (1941). He comments
upon the large amount of yellow fat at the caput epididymis, mentioning
that it may be glandular.
Pedomys Baird 1857
I have not examined the glans in detail, but it does not appear
to differ from that in Microtus (sensu strict) in any appreciable
manner. The baculum has been described by Hibbard and Rinker (1943),
Hamilton (1946), Burt (1960), and Anderson (1960).
The preputial glands lie on the sides of the glans penis. In
the specimen examined they were small (5 x 2 mm), although I am not
certain that this represents maximum development. Usually the
preputials of the Microtinae are very well developed.
The simple tubular vesicular glands are irregular, lumpy, and seem
to represent an intermediate condition between the recurved and con-
voluted types. They are slightly recurved at the terminal portion.
A single gland, when straightened, measured 12 x 3 mm. A single duct
drains each vesicular gland.
There is little glandular tissue of the ampullary glands on the
ventral surface of the vas deferens, most of it being concentrated on
the cephalic, dorsal, and caudal aspects.
There are four pairs of prostate glands. A median, bulbous pair
of ventral prostate glands is situated caudad to the bladder, the ducts
entering the ventral wall of the prostatic urethra at this point. Each
member of the pair is composed of two lobes (a total of four lobes in
this caudad of the bladder). The ducts are confluent, and the nature of
the tubules of the glands is identical. I, therefore, recognize only a
single set of glands in this position. A second, more lateral pair of
ventral prostate glands is located alongside the neck of the bladder,
the ducts entering the prostatic urethra lateral to the neck of the
bladder. All ventral prostates are white in FAA and formalin. The
dorsal prostate glands lie at the antero-dorsal border of the prostatic
urethra, their cephalic edges encroaching upon the dorsal aspect of the
vesicular glands. The ducts of these prostates enter the dorsal vall of
the urethra. The colon passes between these paired glands. The
anterior prostate glands lie along the ventral and lateral aspects of
the vesicular glands. Each gland is loosely bilobed, the longest lobe
(7 x 2 mm) being situated dorsally and projecting more cephalically.
The smaller (3 x 1 mm) is ventral to the larger. These glands empty
into the urethra by means of two ducts on each side (one from each lobe).
which lie adjacent to the ducts of the vesicular glands.
The testis of the specimen examined is small (12 x 7 mm), the
cauda epididymis lying adjacent to the posterior pole. All parts of
the epididymis are invested with yellow fat.
Pitymys McMurtrie 1831
The glans penis of this genus, although not examined in detail,
does not appear to differ from that of Neofiber or Sigmodon in any
salient manner. The bacula of the following species have been described:
P. Pinetorum (Hamilton, 1946; Anderson, 1970; Burt, 1960), P. pyrenaicus
(Didier, 1954), P. quasiator (Anderson, 1960; Burt, 1960), .. fatioi
and arr lus (Anderson, 1960). Judging from the comments and
illustrations of these authors, considerable inter- and intraspecific
variation occurs in the baculum of this genus.
The preputial glands lie on the sides of the glans penis and are
drained by a single duct on each side. These glands encroach slightly
upon the M. rectus abdominis.
The vesicular glands are large and irregular. They cannot be
described as either convoluted or recurred. The terminal portion is
lobate. A single gland measures 10 x 4 mm. The terminal lobes are
depressed ventrally, but do not then curve posteriorly. Each gland is
drained by a single duct that enters the prostate adjacent to the vas
The ampullary glands lie in the usual position at the junction
of the vasa deferentia and each is adnate to its corresponding vas
deferens. The glands are divided into two major portions, lying massed
on the cephalic and caudal aspects of the vas deferens. There appear
to be several ducts associated with each ampullary gland.
There are four pairs of prostate glands. A median, bulbous
(4 x 4 mm) pair of ventral prostates lies caudad of the bladder, the
ducts entering the ventral wall of the urethra posterior to the neck of
the bladder. A second, smaller pair of glands (2 x 1 mm) lies lateral
to the neck of the bladder, their ducts entering the prostatic urethra
at this point. All ventral prostates are white in FAA and formalin.
These paired dorsal prostate glands lie at the antero-dorsal border
of the prostatic urethra, their cephalic edges extending upon the dorsal
aspect of the vesicular glands. The ducts of these prostates enter the
hntero-dorsal wall of the urethra. The anterior prostate glands lie
on the ventral border of the vesicular glands. They are drained by a
pair of ducts on each side which enter the urethra adjacent to the
ducts of the vesicular glands.
Clethrionomys Tilesius 1850
I have not examined the internal structure of the phallus of
this in detail, but it does not seem to differ from the typical com-
plex pattern, The trident-shaped baculum of this genus was figured
by Hamilton (1946). He noted the keel on the ventral surface of the
median process (he referred to this as the dorsal surface) that sepa-
rates the lappet tips of the urethra, though he did not mention its
relation to this structure. Burt (1960) figured this element, but
did not observe the keel ("spur" of Hamilton and Burt). The bacula of
the following species have been figured: C. glareolus (Ognev, 1950;
Didier, 1954), _. rutilus (Ognev, 1950; Anderson, 1960), rufocanus,
S, gapDeeri C. occidentalis (Anderson, 1960).
The vesiculars are large, recurved glands, irregular and somewhat
lobate along the greater curve. The inner margins are heavily lobate
(Fig. 56, B). They are drained by a single duct from each gland that
enters the anterior border of the prostatic urethra adjacent to the vas
deferens. Of the microtines examined, Clethrionomys has the most
heavily recurved vesicular glands. This genus, coupled with Microtus,
stands in marked contrast to other microtines with large convoluted
vesicular glands, though in other details of the tract all microtines
The ampullary glands surround the vas deferens, opening into the
latter through a number of ducts prior to the ampulla. In the specimens
examined these glands had a dark purple color. The original preservative
is not known and whether the color is an artifact or not is uncertain.
Clethrionomys is the only form studied in which the ampullary glands
were such a deep color in preservative. In some of the other genera,
these organs were a pinkish color, but never a deep purple. It is
interesting to note that the lateral pair of ventral prostates had
color identical to that of the ampullaries. A secretary similarity is
possibly suggested by this.
Four pairs of prostates are recognized. A small, 3 x 5 mm,
bulbous, median pair of ventral prostates lies directly posterior to
the bladder. These glands are white in the preservative. The ducts
from these enter the ventral wall of the prostatic urethra posterior
to the neck of the bladder. A second, more elongate (10 x 4 mm) pair
of ventral prostates, more irregular than illustrated, lies lateral
to the bladder. The ducts of these glands enter the prostatic urethra
lateral to the neck of the bladder. These are the same purple color
as the ampullary glands. A large, leaflike pair of dorsal prostate
glands (8 x 5 mm), lies on the antero-dorsal border of the prostatic
urethra through which the ducts of this gland pass at that point. The
colon passes between the two lobes of this gland. The anterior prostates
lie in the lesser curve of the vesiculars, either laterally or medially
depending on the nature of the recurring of the corresponding vesicular
gland. Each anterior prostate gland is drained by two ducts that enter
the prostatic urethra ventral and lateral to the vesicular ducts.
The testis measures 13 x 8 mm. The cauda epididymis almost "caps"
the posterior pole of the gonad, but lies a bit to one side. The tunica
of the testis is very thin, the seminiferous tubules being readily
visible. The testis is pushed in by both cauda and caput epididymis,
possibly because the tunica is so thin. The entire epididymis is
invested with greenish fat.
Rattus Frisch 1775
The phallus is of the complex type and the surface of the glans
is spined. All of the accessory circulatory structures described for
Sigmodon are present. The nature of the glans penis, its apparent
functional morphology, and accessory structures does not seem to differ
in any significant manner from that of cricetids possessing a complex
phallus (Fig. 10). Greene (1935) discusses the origin of the phallus
as well as its innervation and circulation. The most comprehensive
osteogenic work on the baculum of any mammal was done by Ruth (1934)
on Rattus, Although this work is quite comprehensive concerning the
embryology of the shaft, the distal processes are scarcely considered
and no observations were made on the definitive morphology of the
bone. Greene (1935) figures the bone in association with nerves and
blood vessels of the penis, but does not mention the penile blood sacs
and associated circulatory structures.
The accessory glands have been well studied in Rattus, as would
be expected in an animal so frequently used in experimental research.
The majority of the work on this species has been concerned with the
effect of removal of certain prostate lobes, or effects of hormone treat-
ments. Walker (1910) described the anterior prostates as "coagulating
glands," and disclaimed any connection between these and other prostates
(p. 19). Price (1936) considered these glands to be members of the
prostatic series, as do most modern workers, although the name "coagu-
lating gland" is still frequently applied. Snell (1941) discussing
Mus, points out the prostatic nature of these glands.
All accessory glands have been figured, grossly and histologically,
and the ducts of each gland described by Price (1936), and previously
by Walker (1910), in part. Further description is unnecessary. I
recognize an additional pair of prostates probably of the ventral
series, based on vhat appears to be a separate duct system.
The same possibilities of a higher occurrence of morphological
aberrations in inbred strains of Rattus was discussed under Mesocricetus.
Mus Linnaeus 1758
A very adequate gross description and histology of the male
genitalia of Mus is given by Snell (1941). The only discrepancy
between my observations and those previously reported is that I
recognize two pairs of ventral prostates (as I do for Rattus) where
Snell and others note only one. The phallus is of the complex type,
but it has not been studied in detail.
The same conditions of possible morphological aberrations
produced by inbreeding exist in Mus as discussed for Mesocricetus.
The Use of Genital Structures as Taxonomic Criteria
In any taxonomic study one comes to a point at which various
characters must be weighed as to their fundamental or superficial
nature. It would be useless to claim that genital morphology is more
stable per se than cranial or dental morphology which furnish many of
the criteria used in mammalian taxonomy. Indeed, it has many drawbacks.
The genitalia are more difficult to work with than are teeth, and
their nature does not allow definitive measurements to be made. There
is the further difficulty due to age and seasonal variation. Suitable
characters (i.e., ovarian histology ani placentation) may exist in the
females, but are not necessarily comparable to the characters known in
the males. An additional problem is the lack of fluid-preserved
specimens in most museum collections. Soft parts of most mammals,
other than our most common native forms, are difficult to obtain.
A difficulty inherent in the usage of the anatomy soft parts for
taxonomic criteria, is well stated by Vood and Patterson (1959):
". . the evidence of structures that there is, and presumably always
will be, no actual evidence of evolutionary sequence. ."
Any classification based on soft parts may thus be contrary to
the fossil record. This is a problem which can only be solved by more
comparative anatomical studies, dealing with both soft and hard parts,
until some compromise is reached.
Mossman (1953) has pointed out that the degree of plasticity in
any set of structures would theoretically be proportional to the degree
to which those structures interact with the environment. Of all
characters used in mammalian taxonomy, none interact with the environ-
ment more closely than the teeth. Every environmental shift could
presumably bring about some change in diet that might be accompanied
by different selective pressures on the dentition. Then, too, the
degree of convergence and parallelism that is known to exist in some
mammalian groups, as is the case with the rodents (Simpson, 1945;
Wood, 1950; 1955) leads one to be cautious of classifications based
solely on dental patterns.
It is difficult to see how the genitalia could be influenced
to any great extent by environment, though there may be some association.
The best statement of the supposed conservative nature of the genitalia
was made by Mossman (1953), in a discussion of the fetal membranes,
which is also perfectly applical to genital characters in general:
What then should one expect of the evolution of an organ
system, such as the fetal membranes, largely isolated
from the external environment and charged with a highly
complex job to do, but one which is fundamentally the
same in all members of the group, in this case the
Subclass Eutheria? It seems that, regardless of rate of
genetic variation of the system, divergence would be
relatively slow and narrow, and that intergradation would
be the rule, there being slight environmental selection,
and little race isolation due directly to adaptive
variations in this system itself. But while this type
of evolution would be taking place in an environmentally
independent organ system, the group of organisms in which
the organ system existed could nevertheless be undergoing
the usual evolutionary course in its other organ systems:
diverging widely in adaptation to environmental niches
old and new; losing species or whole major groups by
extinction; in short, becoming the widely divergent,
often aberrant or isolated groups that are characteristic
of mammals at the present time.
Despite such considerations, one must also consider that the
large number of phallic forms, especially the great variation found
in those of the simple type (Hooper, 1958, 1959, 1960), must be the
result of selective forces, either direct or indirect. To what degree
this is comparable to the selective forces operating on dental or
osteological characters is, of course, unknown.
It is unfortunate that so little information is available on
the embryology of many of the reproductive characters Until such
information is obtained, many of the homologies of various prostatic
series cannot be determined with assurance.
Of great concern in the taxonomic evaluation of the genital
characters discussed here, is a consideration of what degree of
genetic divergence is necessary to produce different glandular comple-
ments, and to what degree minor deletions of these complements affect
the animal in question. Thus, if the secretary role of a particular
prostatic pair is either dispensable, or can be carried out by another
pair of glands (prostatic or otherwise), any mutation producing the
reduction, or elimination, of the glands in question would not be
strongly selected against. As was previously mentioned, inbred
strains of Rattus and Mesocricetus do demonstrate aberrant character-
istics, showing, perhaps, that in certain populations genital variants
may become established.
Until much more information on the embryology, secretary nature,
and genetics of such characters is available, it is best to consider
genital characters as potential taxonomic characters in certain groups,
but certainly of no proven stability or freedom from selection.
Patterns of Morphological Variations in the Male
Genital Tract of Muroid Rodents
Structure of the phallus.-Two types of phallic structures,
simple and complex, are clearly evident among muroid rodents. Such
fundamental diversity as exists between these two types of glans
is of an impressive nature.
The complex form of phallus appears to be too intricate a
structure, and too similar in the various forms possessing it, to have
been evolved independently in various phyletic lines (Figs. 6, 8, 9,
and 11). It must therefore represent a fundamental pattern,
characteristic of both miuid and cricetid rodents, perhaps even the
Myomorpha. The simple type phallus, occurring in New World forms
only, would appear to be derived from the complex form in the
following manner: (1) reduction in the complexity of accessory circu-
latory structures and elongation of the shaft of the baculum. This
would have to be accompanied by a functional change as outlined on
page 14, resulting in the functional natures of the two types of
phalli; and (2) simplification and final elimination of the digital
processes of the distal end of the baculum. As the digital processes
are movable in the well developed complex phallus, and dependent
upon the accessory circulatory structures for this movement, their
regression following a loss of the latter structures would seem inevi-
table. The manner of formation of the dorsal vein of the penis in forms
possessing a simple type phallus (Figs. 9 and 10) seems to have retained
the nature of that of the complex type, though the penile blood sacs,
present in the primitive phallus, are not present.
Two genera, Akodon (Fig. 9) and Nyctomys, represent possible
stages in this development. In both genera the conditions for
reduction are met. This is not to say that these genera are ancestral
to certain modern genera with simple phalli, but that they demonstrate
possible stages in the development of the simple type. They may be
ancestral, or they may represent separate lines paralleling those
which have already developed the simple type of phallus.
Accessory glands.-The evidence of two distinct lines is not as
clear from the accessory glandular structures as from the phalli.
Certain genera of each of the two phallic forms have lost particular
series of glands. If the nature of the phallus is taken to represent
familial or subfamilial distinction, the accessory glandular complement
would logically be subordinate to this level, possibly at the tribal or
supergeneric rank. Here, one must weigh the frequency with which a
particular set of glands is lost or highly modified. If one gland set
has been dropped several times by several groups of animals, it would
appear to be of a less fundamental nature from a taxonomic standpoint,
than a gland set that has been retained by all but one group.
The preputial glands have been lost by numerous genera (Table 2),
some with a complex phallus, some with a simple phallus. Lack of
preputial glands is generally associated with the simple type phallus.
This is true of Peromyscus, Reithrodontomys, and Neotoma, but three
genera, Ochrotomys. Onychomys, and Baiomys. have both a simple phallus
and preputial glands. I have not examined the other six genera with
simple types of phalli listed by Hooper (1960). In the seven genera of
microtines examined preputials were large and conspicuous. No forms
SUMMARY OF THE VARIATION IN ACCESSORY GLANDS
OF 24 MUROID GENERA
Phallus Preputial Bulbo-Urethral Ampullary Vesicular
Dorsal Ventral Anterior
1 1 1
1 2 1
Phallus: C = complex type
S = simple type
X = structure present
0 = structure absent
* = 2 pairs present
Phallus Preputial Bulbo-Urethral Ampullary Vesicular
Dorsal Ventral Anterior
1 2 1
examined demonstrated as high a development of these glands as did the
microtines. Of the cricetines with a complex phallus type, only
Mesocricetus lacked preputial glands. The status of Phyllotis and
Tylomys in this respect could not be determined because of incomplete
specimens. Two genera, Akodon and Sigmodon, have two pairs of pre-
putial glands, a small ventral and a large lateral pair.
The bulbo-urethral glands exhibit the greatest uniformity of all
accessory glands. With but minor variations these glands occupy the
same position and have similar duct systems in all genera. These are
the most stable accessory glands in the reproductive tract of muroids,
possibly of mammals in general.
With the exception of Nyctomys, all genera with a complex phallus
had vesicular glands. Of the genera with simple phalli, only Neotoma
and Onychomys lack vesiculars. Baiomys and Onychomys have vesiculars
that are reduced. Only Peromyscus and Reithrodontomys of the genera
with a simple phallus type had well-developed vesicular glands.
The fully developed vesicular glands are quite large in most
genera. They are easily the largest glands of the tract. Basically
the vesiculars are simple tubular glands, though when distended they
become thrown into numerous convolutions. These may be of two basic
types. In cricetines and muroids, they become recurved, and highly
irregular on the greater curve of the gland. In microtines, and those
cricetines in which the glands are reduced, the tubular nature of the
glands remain apparent, and numerous bends, apparently fortuitous, are
made. In these particular forms, the convolutions of the glands are
evidently determined by the location of other internal organs. Ondatra
and Neofiber best illustrate this condition. Clethrionomys has a more
generalized, recurved type, and Microtus, Pitymys and Pedomys are
The ampullary glands are rather constant in shape. In Baiomys,
these are reduced to several small tubular glands, unconnected, draining
into the respective vas deferens. Ochrotomys and Onychomys lack these
In several genera, particularly Nyctomys and Neotoma, the
ampullaries are large and filiform. This may be due to the absence of
the vesiculars, which generally form the anterior border for these
In Tylomys a most distinctive condition exists. These are truly
"ampullary glands," draining into the ampulla directly. All others
enter the corresponding vas deferens, as well as the ampulla, by numer-
ous ducts. In larger forms (e.g., Ondatra) the ampullaries are con-
spicuously lobed. Whether this is a response to the large size of the
animal alone, is not known.
The prostate glands vary in form and number of pairs among the
various genera. The dorsal prostates occur in all but two genera
(Nyctomys and Neotoma). The ventral prostates are represented by either
one pair (seven genera), two pairs (16 genera), or are absent (Nyctomys
only). If two pairs of ventral prostates are present, one (lateral)
usually assumes a red color in FAA, the other (median) remains white.
Although this can probably be safely attributed to the nature of the
secretions of the respective glands, no further information is available.
The anterior prostates are present in 22 of 24 genera studied. They
are absent in Tylomys, and Onychomys. In two other genera, Neotoma
and Nyctomys. they are greatly enlarged and modified. This modification
of the anterior prostates occurs in the only two genera in which the
vesicular glands are also absent. In Ryctomys. these are the only
prostates that occur.
The homologies that I have attempted to ascertain in describing
these prostate glands are based on gross morphology and duct systems.
The problem exists, though, that if a basic "prostatic complement"
consists of (let us say), three pairs, evolution from this stock may
result in several forms having only two pairs: and it is not possible
at present to determine precisely which pairs have been lost in all
cases. The nature of the glandular secretions would be of much use to
the taxonomist as well as interest to the physiologist. How can the
sperm of presumably closely related animals such as Onychomys and
Peromyscus. exist in semen which is produced by two prostates and the
bulbo-urethrals in one form (Onychomys), and by four pairs of prostates,
vesiculars, ampullaries, and bulbo-urethrals in the other (Peromyscus)?
What is the fundamental importance of these glands? If one genus has
five sets of prostates and another only one, does the one of the latter
genus produce all that is produced by the five of the former? Perhaps
the female tract produces those substances not produced by the male. We
must admit that the physiological consequences of reduction, loss, or
addition of accessory glands is totally unknown.
In addition to secretary problems involving the prostates, the
fatty tissue with which the epididymides are invested is of interest.