526 
MR. OWEN ON THE MAMMARY GLANDS 
above mentioned being little more than a change in the character of the lining 
membrane. The urethro-sexual canal, on the contrary, opens into the vesti¬ 
bule 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 rectum there are from six to eight small apertures of dark- 
coloured glands or follicles, about the size of a pin’s head, situated immedi¬ 
ately behind the proper membrane of the vestibule. 
The urethro-sexual canal is one inch and a half long, and three or four lines 
in diameter, but capable of being dilated to as great an extent probably as the 
pelvis will admit of, the diameter of that passage being seven tenths of an inch. 
It is also invested with a muscular coat, the external fibres of which are lon¬ 
gitudinal ; the internal, circular. The inner membrane of this part was dis¬ 
posed in longitudinal rugae more or less marked, but presented as little the 
character of a secreting membrane as that of the vestibule, being smooth and 
shining; after a careful examination with the lens, the orifices of a few minute 
follicles were discovered in the interstices of the rugae near the orifice of the 
urinary bladder. 
It is this division only of the passage from the uterus which is situated within 
the pelvis, the vestibule being produced beyond it, and the common outlet 
being in consequence situated at a considerable distance from the outlet of the 
pelvis, as in the beaver, which besides its analogy in habits to the Ornithorhyn- 
chus is also in the literal sense of the word monotrematous. If, then, the Or- 
nithorhynchus be really oviparous, its eggs must be disproportionately small 
compared with those of birds, in order to pass through the pelvis. For on the 
supposition that they are of “ the size, shape, and colour of those of a hen *, ” 
the yelk at least must be much smaller; for it is obvious that this part only of 
such an egg could pass through the pelvis, and the albumen and shell must 
necessarily be laid on in the 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 a secreting membrane; and great alterations 
at least must take place in them, if they exercise any share in contributing to 
the nutrient store of the embryo. 
At the atlantal extremity of the urethro-sexual canal there are five distinct 
* Linn. Trans, vol. xiii. p. 624. 
