280 ' WILBUR WILLIS SWINGLE 
later upon the entire developmental history of the male and 
female sex glands and cells in anurans. The problem stated in 
the heading of this section is of such importance as to deserve a 
much more detailed discussion than is possible here. However, 
a brief statement of the more important theoretical considera- 
tions suggested by the results obtained in the study of the larval 
sexual cycle of the bullfrog cannot well be avoided. There are 
certain obscure and little-known phenomena occurring in several 
other classes of vertebrates of a similar, if not identical nature 
with those reported here for the Anura, When the germ-cell 
cycle of some of the higher vertebrates is correlated with the 
maturation cycle of the larval frog, there is much that suggests 
to the writer that possibly we are here dealing with a fundamental 
principle in germ-cell development, of widespread, perhaps of 
universal occurrence among the vertebrates. Let us examine 
some of this evidence. 
There are two important theories concerning the origin of the 
germ cells of vertebrates, each backed by considerable amounts 
of evidence not hghtly to be disregarded: 1) The first view is 
that the definitive sex cells of the gonads are derived from pri- 
mordial germ cells which have originated elsewhere in the organ- 
ism, probably from entoderm, and have migrated into the genital 
ridges and there differentiated into oocytes or spermatocytes 
as the case may be. These primordial germ cells of the embryo 
are distinct from other surrounding mesothelial cells and have a 
separate origin from cells of this type. The advocates of this 
view are many, and a great deal of valuable data in support of 
this theory has been collected of late years. 2) The second view 
regards the germ cells as differentiated products of the germinal 
epithelium, i.e., that they arise by direct transformation of sexu- 
ally indifTerent cells of mesodermal origin. 
The advocates of the first point of view, we may, for the sake 
of convenience, term the 'entodermists,' though not all behevers 
in the Keimbahn theory are agreed that the germ cells take origin 
from entoderm; the adherents of the second theory we shall call 
'mesodermists.' Between these two groups there is little or no 
common ground, but instead a great deal of controversy. 
