288 WILBUR WILLIS SWINGLE 
Another point is worthy of consideration here, and that is the 
possibihty of bringing about some measure of reconcihation be- 
tween the 'entodermists,' or advocates of the Keimbahn, and the 
'mesodermists.' In view of the evidence presented by study of 
germ-cell origin in all classes of vertebrates, there can be no 
reasonable doubt that the primordial sex cells are products of 
the entoderm, and probably migrate into the germ ridges at an 
early period of development. However, according to the hypoth- 
esis advanced here these primordial cells, after a period of mul- 
tiplication, undergo an abortive developmental cycle and for the 
greater part degenerate — perhaps, in mammals, entirely degen- 
erate. The new cell generation destined to give origin to the 
definitive sex cells may possibly arise in part from the germinal 
epithelium by direct transformation of mesothelial elements. 
The evidence for this point of view is suggestive, at any rate, 
judging by reports on conditions in the birds and mammals, 
and there is little evidence to the contrary, but many pure 
assumptions. 
In the bullfrog the writer prefers to believe that some cells of 
the primordial germ-cell line persist unchanged through the phase 
of maturation and degeneration, and ultimately, by repeated 
mitosis are the chief, and probably only contributors to the cells 
of the definitive sexual hne. There is considerable evidence for 
this view, because a few primordial spermatogonia or at any rate 
lineal descendants of these cells can be traced through the sexual 
cycle easily enough, but it is by no means certain that they are 
the sole contributors to the definitive line of germ cells. 
Thus it appears possible that there is some basis here for rec- 
oncihation between the entodermists and mesodermists regarding 
germ-cell origin and development. The former have been at fault 
by contenting themselves with tracing the primordial sex cells 
into the genital glands and there leaving them, with the assump- 
tion that they persist and form the sexual elements of the adult 
organism. The mesodermists, working chiefly on mammals, 
have for the most part ignored the contributions of the 'ento- 
dermists' because they have been unable to trace the germ cells 
back to the very earhest stages such as described for the lower 
vertebrates. 
