REPRODUCTIVE HISTORY OF THE MOLE. 213 
female shows but little advance upon the condition described as 
existing in the nestling of 59 mm, ‘The utero-vaginal canal is a 
wide cavity which narrows below the neck of the bladder and 
passes, still as an open tube, towards the perineum at the base 
of the genital tubercle. But at a point some distance from the 
perineal surface it becomes closed by the approximation of its 
anterior and posterior walls, and is continued as a wide area of 
epithelial cells which becomes continuous with the skin at the 
base of the genital tubercle (text-fig. 11, p. 211). 
There is, therefore, no outlet for the female genital passages, 
for the whole of the lower part of the tract is a solid epithelial 
mass, just as it is in the 59 mm. nestling. 
The greatest advance to be noted in the adult examples of 
which I have prepared serial sections, is that the solid epithelial 
area is of greater lateral extent and becomes continuous with the 
whole of the lower, patent, extremity of the utero-vaginal canal. 
In the pigmented young females taken in April a very great 
change is seen. 
All parts of the genital tract are much enlarged, their tissues 
are all more vascular, and the large pre-anal gland mass has 
increased greatly in size. But the most important change 
consists in the complete opening of the lower part of the utero- 
vaginal canal by the desquamation of the central cells of the 
previously solid epithelial mass (text-figs. 12, 18, A). So far as 
can be determined from the material at my disposal, it would 
appear that with the increased vascularity of the parts in the 
months of March and April, a proliferation of the epithelial mass 
is initiated, that the central cells of the mass degenerate and are 
shed, and the peripheral cells become the lining epithelium of 
the cavity thus opened up (text-fig. 13, B). 
In sections of the genital tract of non-virgin females caught 
in the autumn months the retrogressive changes are seen. The 
lower portion of the utero-vaginal canal is again occluded, but 
the occlusion is not uniform throughout the epithelial portion. 
Pockets run in from the surface, and in some cases these are deep 
enough to admit the tip of a probe, but beyond these pockets the 
occlusion is again complete in whole or in part. 
F. Summary and Conclusions. 
(a) Although the peculiarities of the female reproductive 
system of Zalpa ewropea appear at first sight to be extremely 
anomalous, nevertheless they have, for the most part, some 
foreshadowings in the normal processes of development of other 
mammalian types. 
(0) Everything that is unusual concerning the female repro- 
ductive system is initiated in the early stages of embryonic 
development when, between the 9 mm. and 18 mm. stages, the 
labio-scrotal folds begin to grow towards the middle line over 
the closed urethra in the base of the genital tubercle. 
