OVARIAN FOLLICLES IN FERRETS AND FERRET-POLECAT HYBRIDS. 323 
rounded them were the cells of the tubes and nests, gradually isolated into follicles 
by the interposition of stroma cells. Further, it was asserted, and generally believed, 
that all the ova which come to maturity were formed in the ovary during the 
embryonic period ; elaborate calculations of their numbers were made, and the 
results are still at times quoted. 
After “ germ cells ” were discovered, and their migrations from their seats of 
origin to the ovary were traced, the assumption previously adopted was modified, 
and it became usual to accept the “germ cells” as the progenitors of the ova, 
whilst the follicle cells were considered to be the descendants of the germinal 
epithelium which covers the surface of the ovary. Definite proof that ova are 
derived from “germ cells” in mammals has never been produced, but the sugges- 
tion was enticing, and it received ready acceptance, although it was unaccompanied 
by any explanation of the reason why of all the ova and follicles, formed at 
approximately the same time, some proceeded to further growth and development 
at relatively long periods, in some cases years, before the others, whilst all were 
presumably subjected to the same influences at all periods. 
In 1905 Miss Lane-Claypon (16) made a series of observations, on the ovaries 
of rabbits, which led her to propound the conclusion that the ova, the follicle cells, 
and the interstitial cells of the ovary are all formed by the germinal epithelium, 
and that the interstitial cells are potential ova. Her general conclusions have 
not been confirmed by other observers, but it has become increasingly evident 
from the researches of Winiwarter and Sainmont on the cat (43), Rubaschkin on 
the guinea-pig (33), and Kingery (15) on the white mouse, that the ova formed 
by the early embryonic proliferation, whether they are descendants of the germinal 
epithelium cells or of special germ cells, are not those which attain to maturity and 
become the rudiments of a new generation of animals, for they all die before they 
have attained any advanced stage of development, and many of them before they 
become enclosed in follicles. 
According to Rubaschkin (33), the ova which attain to maturity are formed by 
a proliferation of the cells of the germinal epithelium which takes place before birth, 
but which is, at all events, a third and not a first generation ; whilst Winiwarter and 
Sainmont (43) found, in the cat, that the ova which reach maturity are formed by 
a proliferation of cells, starting from the germinal epithelium and taking place from 
three and a half to four months after birth. If this is the case, explanation is still 
wanting as to why certain numbers of the follicles and their contained ova mature 
for each ovulation period ; for all being formed at the same time are of the same 
age, are situated in practically similar situations, and are subjected to the same 
influences which bring about the maturation of only a small number at any given 
ovulation period. 
It seems improbable that of a large number of ova and follicles formed at the 
same time only a few shall attain maturity at an early epoch, whilst the majority 
