MONOTREMATA. 



395 



vestibule. But, as has been before observed, 

 neither the lining membrane of the vestibule nor 

 that of the genitourinary 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 (b) 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 raphe 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 approximate these orifices ; and in this 

 action the oblique fibres above described would 

 assist, while at the same time they closed 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- 

 quently 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- 

 nary 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 clitovidis. 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 b', 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, 

 in one of the specimens examined, made a small 

 circular and valvular projection into that cavity. 



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. A yolk of large 

 size appears, however, to be an essential con- 

 dition of an egg which is to be hatched by a 

 warm-blooded animal out of the body ; but 

 the vitellme 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, 

 chalazae, and cortical investments of the bird's 



egg- 



The unimpregnated ovarian ovum from an 



ovisac of two lines in diameter, of the Orni- 



