290 WILBUR WILLIS SWINGLE 
vertebrates, and that there is no actual transformation of meso- 
thehal elements into sex cells, the writer admits that conditions are 
such in the bullfrog that it is impossible to state positively that the 
primordial germ cells of the bullfrog tadpole do give rise to the 
definitive sex cells of the adult frog. Certainly, this is the more 
probable view, though the burden of proof rests with those of us 
who hold that the keimbahn is continuous. 
It would seem from this that the crux of the whole problem is 
to determine whether or not germ cells can develop in an organ- 
ism after the primordial germ cells have been destroyed. If 
they do develop, then the doubtful question of transformation 
of mesothelial cells into germ cells is settled in favor of the meso- 
dermists, but if they do not develop, and the gonad is sterile and 
remains so up to the period of sexual maturity, then the decision 
is in favor of the entodermists. It is not sufficient to extirpate 
the primordial germ cells or otherwise destroy them, as was done 
by Reagan in the embryo chick, and then report the resulting 
sterile gonad as conclusive evidence against the idea of a trans- 
formation of epithelial elements, because proof positive can only 
be had by rearing the animals to sexual maturity. 
The only adequate method of attack upon this problem is by 
experimental methods. Morphological methods are not sufficient 
to determine whether or not a germ cell in the germinal epithelium 
or sex cord tissue is a transformed epithehal element or a small 
germ-cell descendant of the primordial line. Transition stages, 
nuclear configuration of the cell, size, position, and such like may 
be illusory. In my material there is apparently every transition 
stage between peritoneal and true germ cells, use whatever 
morphological criterion you please, at certain developmental 
stages of the tadpoles, and such transition stages almost fill the 
gonads, but always the question arises upon examining these 
apparent transition stages — they look exactly like mesothelial 
cells transforming into germ cells, but are they? If one must 
judge from morphological data alone, the answer is that they 
could very readily be taken for mesothefial elements transforming 
into germ cells, but, as stated before, the morphological criterion 
alone does not furnish sufficient evidence to permit one to make 
a definite answer. 
