282 WILBUR WILLIS SWINGLE 
derm in the male bullfrog larva undergo a precocious and abor- 
tive maturation cycle ending in degeneration and absorption. A 
very few descendants of the primordial line of cells fail to maturate 
or degenerate, and apparently give rise to a new generation of 
sex cells in the tadpole. Whether or not this second generation of 
germ cells is derived entirely from the few left-over cell descendants 
of the primordial line is difficult to say, for in my material at this 
time there is a marked increase of germ cells which looks suspic- 
iously like an active transformation of epithelial elements into sex 
cells. The new germ-cell generation undergoes another matura- 
tion cycle at the time of the metamorphosis of the tadpole, or 
very shortly afterward, and gives rise to normal sex products. 
Thus it is to be seen that the advocates of the germinal epithelium 
theory may not be entirely mistaken, for in the light of events 
in the germ cycle of the Anura, it is not sufficient to trace the 
primordial sex cells into the genital ridges and there leave them. 
Without following their later history, it is unjustifiable to assume 
that they do give rise to the definitive sex products. This is 
what most workers on the origin of germ cells in vertebrates have 
done. On theoretical grounds the writer doubts if mesothelial 
cells transform into sex cells but the morphological evidence from 
my material does not rule out the possibility of such transfor- 
mations. 
The evidence presented by the precocious sexual cycle of the 
bullfrog leads the writer to believe that further investigation 
may perhaps show the existence of analogous phenomena through- 
out the various groups of Chordata. It may not be too much 
to state as a sort of working hypothesis for further investigation 
of germ-cell development in the vertebrates, that most of the 
primordial germ cells, i.e., those arising in early stages pi embry- 
onic development, undergo a precocious and abortive develop- 
mental cycle, culminating in degeneration, or else degenerate 
before the precocious maturation cycle has had sufficient time to 
manifest itself. Furthermore, that the abortive developmental 
phenomena are perhaps evidences of a phylogenetic regression 
in the germ-cell cycle to remote ancestral conditions. 
The question immediately arises if such an hypothesis, no mat- 
ter how tentatively stated, is in any sense justified by evidence 
