MONOTREMATA. 
vestibule. But, as has been before observed, 
neither the lining membrane of the vestibule nor 
that of the genito-urinary passage presents the 
characters of an active secreting membrane; and 
it is highly improbable that an almost callous 
surface daily traversed by the excrements, 
should be suddenly modified to contribute so 
important a share to the nutrient store of the 
embryo. 
The common vestibule (5) is about one inch 
four lines in length, and varies from half an 
inch to an inch in diameter. The muscular 
fibres immediately investing it are disposed as 
follows. A thin circular muscle arises from a 
dorsal raphé which extends the whole length 
of the canal. Of this muscle the sacral fibres, 
or those nearest the outlet, surround the whole 
vestibule; but the atlantal or more internal 
fibres pass obliquely upwards, and surround 
the termination of the rectum only, serving as 
a sphincter to it. On the sternal aspect of the 
vestibule there are a series of longitudinal 
fibres, which extend from its external orifice to 
that of the urogenital cavity, the office of which 
is to el elena these orifices; and in this 
action the oblique fibres above described would 
assist, while at the same time they clused the 
rectum. 
On the sternal aspect of the urogenital 
_ canal, and close to where it joins the vesti- 
bule, the clitoris is situated, which is conse- 
- about an inch and a half distant from 
the external orifice of the vestibule. It is 
inclosed in a sheath upwards of an inch in 
length, and about two lines in diameter, of a 
_ white fibrous texture, and with a smooth in- 
ternal surface, and this sheath communicates 
_ with the vestibule about a line from the ex- 
_ ternal aperture. The clitoris itself is a little 
flattened body shaped like a heart on playing 
_ Cards; it is about three lines long, and two 
lines in diameter at its dilated extremity, where 
_ the mesial notch indicates its correspondence 
of form with the bifurcated penis of the male. 
From the shortness of the clitoris, and the 
length of its sheath, it is obvious that no part 
of it can project into the vestibule in the ordi- 
“hary state of the parts, as stated by Sir Everard 
Home, its extremity being situated at least an 
inch distant from where its sheath communi- 
cates with that cavity. 
At the base of the clitoris are two small 
round flattened glands, the analogues of Cow- 
per’s glands in the male, which open into the 
sheath or preputium clitoridis. These glands 
were largest in the specimen whose uterine 
Organs were most developed. 
The vestibule is lined by a dark-coloured 
cuticular membrane, and has a tolerably uni- 
form surface. The rectum opens freely into it 
posteriorly, as indicated by the probe 6’, in 
Jig. 191; the line of distinction in the relaxed 
State of the sphincter above-mentioned being 
‘little more than a change in the character of 
the lining membrane. 
_ The urogenital canal, on the contrary, opens 
into the vestibule by a contracted orifice, and, 
1n one of the specimens examined, made a small 
circular and valvular projection into that cavity. 
395. 
On either side the termination of the rec- 
tum there are from six to eight small apertures 
of dark-coloured glands or follicles, about the 
size of a pin’s head, situated immediately. 
behind the proper membrane of the vestibule, 
and corresponding with the anal follicles of 
the Marsupial and other Quadrupeds. 
The female organs of the Echidna corres- 
pond in all essential points with those of the 
Ornithorhynchus. 
Products of generation.—The mode of ge- 
neration and course of development of the 
Monotremata, although elucidated in many 
essential points by the light of anatomy and 
analogy, still demand observation of the breed- 
ing animals, and of the impregnated uterus 
and embryo in several stages, before they can 
be fully determined. ; 
The close resemblance of the efferent tubes, 
in their complete separation and in their mode 
of termination, with the oviducts of Reptiles, 
and more especially of the Oviparous tortoises, 
added to the approach to the very peculiar 
condition of the female organs in Birds which 
the Monotremes offer in the unequal develop- 
ment of the ovaria and oviducts, have inclined 
several physiologists to a belief in the reports 
which from time to time have been published 
of the discovery of eggs in the nests of the 
Ornithorhynchus. But a comparison of the 
organs themselves with the conditions essential 
to the Oviparous generation of a warm-blooded 
animal offers almost insuperable difficulties to 
this view. The difference of structure and 
dimensions between the uterine and oviducal 
divisions of the efferent canal is greater in the 
Ornithorhynchus than in any bird, and so 
closely corresponds with that which charac- 
terizes the uterus and oviduct in the Kangaroo, 
that it is highly probable they perform similar 
functions in the completion and development 
of the ovum. 
But the experienced physiologist would seek 
to obtain a closer insight into the mode of 
generation of the Monotremes by a comparison 
of the more essential generative organ,—the 
ovary,—with that of the bird. Now it has been 
shown that this organ, at the period of sexual 
excitement, does not differ in size or form from 
that of the Rodent or Marsupial quadruped ; 
nay, that some of the latter tribe, as the 
Wombat, more nearly resemble the bird in the 
large size of the ovisacs or calyxes than the 
Monotremes are known to do. Avyolk of large 
size appears, however, to he an essential con- 
dition of an egg which is to be hatched by a 
warm-blooded animal out of the body; but 
the vitelline portion of the ripe or nearly ripe 
ovarian ovum of the Ornithorhynchus does not 
equal the twentieth part of the yolk of the 
egg of a bird of corresponding size: and it is 
scarcely necessary to observe that the yolk is 
always exclusively a product of the ovarium, 
and that the material added to it by the oviduct 
or uterus must be analogous to the albumen, 
chalaze, and cortical investments of the bird’s 
The unimpregnated ovarian ovum from an 
ovisac of two lines in diameter, of the Orni- 
