286 WILBUR WILLIS SWINGLE 
Kirkham ('16) observed that the primordial germ cells of the 
mouse first appear on the eleventh day after fertilization. In 
male embryos these primitive germ cells all degenerate, and none 
persist by the eighth day after birth. The definitive sex cells 
of the male arise from undifferentiated epithelial elements accord- 
ing to this account, whereas the definitive oogonia are direct de- 
scendants of the primordial germ cells. It will be seen that 
Kingery's account of the definitive germ cells of the female mouse 
agrees with Kirkham's account for the corresponding conditions 
in the male. 
Felix states that in the human embryo the primordial germ 
cells degenerate (no details given) and a new generation of sex 
cells arises from the germinal epithelium which give origin to the 
definitive sex cells (Keibel and jMall, Embryology). 
Now it is obvious that evidence of this sort obtained b}^ dif- 
ferent investigators, working on vertebrate forms as widely sep- 
arated as amphibia and mammals, must be of some significance. 
In all vertebrates a definite Keimbahn probably exists; this is 
certainly true of the frog, but the important question is, do the 
primitive products of the keimbahn and their lineal descendants 
in these vertebrate forms early undergo an abortive maturation 
or developmental cycle which ends in degeneration such as occurs 
in the bullfrog tadpole? Certainly, the evidence looks sugges- 
tive. Apparently in the male bullfrog larvae this precocious 
sexual cycle is carried further than in any other form so far 
reported. The figures of Kingery for the female mouse and of 
Winiwarter and Sainmont for the female cat indicate plainly 
that the primordial germ cells are undergoing a precocious matura- 
tion cycle. These figures show every phase in the maturation 
cycle of normal eggs, such as leptotene, pachytene, diplotene, 
and growth of the oocyte, yet, just as happens in the male tad- 
pole, these early maturating cells degenerate. The same con- 
dition is reported in birds. It is difficult to avoid the suspicion 
that we are here concerned with a fundamental principle of germ- 
cell development. The question arises, why should practically 
all of the primordial germ cells of vertebrate undergo an abor- 
tive sexual cycle long before the animal is mature and ready for 
