Miller. J 466 fJ^"- 'f'- 
m'itluT tlu' vaginal walls nor iiiesodeniiiil tissiK' of any sort. 
It, is thus iiii|iossil)lc to homologizo it witli the hyiiu'ii. 
On the other hand the use of siieli a structure is not far to 
seek. Siiu-e in the Muridae no hibia are present, the vaginal 
orifice is constantly exposed to injuiy through the introduction 
of particles of dust, dirt, and sand. The <levelopment of strong 
sphincter muscles, Uke those which surround the anus, would be 
efficacious in preventing the ingress of foreign bodies, but such a 
method of protection is much less likely to have been established 
for the introitus vaginae on account of the enormous dilatation to 
which this orifice is subjected during parturition. A sheet of 
epithelium sealing the vaginal orifice ;uid capable of reproducing 
itself whenever ruptured is, on the other hand, such a simple and 
efficient protection that its perpetuation is readily comprehended 
from the standpoint of the survival of the fittest. ^ If we go 
farther and inquire what in the first instance may have induced 
in this region a tendency to exaggerated cell growth, an answer 
suggests itself as follows. Since mechanical u-ritation of epithelial 
tissue causes cell proliferation, it may be readily imagined that a 
growth of this character could be originated as the result of the 
action of foreign substances in the unprotected vaginal orifice. 
This tendency to cell growth in a detinite region once established, 
the protection afforded by it, even though incomplete, might 
offer sufficient opportunity for the operation of natural selection, 
whereby the detinite and useful structure that we now find could 
be perfected. 
Cambkidge, Dkcembek, 1894. 
1 E|)itliL'li;il t;ruvvths of :i somewhat aiiaUifious rliarauler are met with elsewliL'ie. 
Durint;- the [leriod when the young of certain niaisiipials adhere to the nipples within 
tlie pouch the lips are grown together by a proliferation of ei)itlielial cells. Thus 
the mouth of the helpless young animal is temporarily converted into a tube closely 
moulded to the nipple of the parent and furnishing a mean.s of secure attachment 
(Leche, ".)0). 
Kohl ("J3) states that at one stage in the (levclopmcnt of the eyes in Talpn mropaca 
the lids are closed by a plug of epithelial cells which, however, are resorbed before 
birth. The appearance of this plug, as shown by Kohl's figures, bears a certain 
superficial resemblance to the cells of the Malpighian layer in the mass of thickened 
eiiithelium at the introitus vaginae in Peromyscus. 
The temporary fusion of the eyelids in young animals, a condition which generally 
(li8ai>i)ears just before or immediately after birth, is a well-known fact, whose 
signilii-ance in the present connection should not bo lost sight of. 
