REPRODUCTIVE HISTORY OF THE MOLE. 199 
their invading margin (text-fig. 2, B & C). The anus has become 
circular in outline and its margins have become more distinctly 
separated from the skin which forms the perineum, for although 
by no means so protuberant as in the adult, the anus of the more 
mature embryo is situated upon the apex of a conical tumid 
mound which rises from the general surface of the perineum. 
The genital tubercle ensheathed in the overgrown outer genital 
folds has become recognisable as the adult penis, and between 
this penis and the anus the prominence of the outer genital folds 
has diminished to make a more or less flattened area—the adult 
perineum. By full term (30-40 mm.) the genital tubercle is 
completely ensheathed, and hidden from view, within a prepuce 
derived from the outer genital folds; the anus is situated at the 
summit of a conical elevation of skin, and the perineum shows 
neither a median raphé nor an elevation due to the presence of the 
outer genital folds. The naked nestling shows the same condition 
with all the parts considerably enlarged (text-fig. 3). 
The stages that have been described take place in exactly the 
same manner in both sexes; so it seems obvious, from external 
examination, that in the female, as well as in the male, the 
urogenital sinus must be carried forwards, by closure of the inner 
genital folds, to the tip of the genital tubercle, which then 
becomes secondarily ensheathed within the’ overgrowing outer 
genital folds. In other words, it appears as though the outlet for 
the genital products in both sexes must be at the extremity of 
the penis or clitoris. 
C. Post-natal Stages of External Genitalia. 
As Thomas Bell truly said, “it is very difficult to know a 
virgin female mole from the male.” 
Moles breed but once a year, and an extended series of obser- 
vations by Adams and others proves that in the great majority 
of cases the three or four young are born during the month of 
May. 
These young grow very rapidly. According to Adams (7) they 
double their length within the first ten days of their life; three 
weeks after birth they are three-quarters grown (H. E. Forrest, 8), 
and by the later months of the year they resemble the previous 
year’s adults in all obvious characters. 
I started collecting moles for examination in the autumn 
months; and from among the many individuals secured it was 
not difficult, after some experience, to pick out the females of the 
year. 
Text-fig. 4 shows the condition of the external genitalia present 
in these young females. The particular specimen figured was 
caught in November, and it shows but little modification of the 
late foetal and early nestling conditions already described. Text- 
fig. 5 shows the condition of a male taken at the same time, and 
