No. 2] . THE OOGENESIS OF BUFO LENTIGINOSUS. 373 
In Bufo, as in the turtle and in the frog according to the 
investigations of Allen (1, 2), the germ-cells arise in con- 
nection with the endoderm. Allen’s recent account of the 
origin of the sex-cells in Rana pipiens agrees essentially with 
what I have found in Bufo. I cannot be sure, however, that 
in Bufo the ridge of germ-cells is separated. from the endoderm 
“by the approximation of the lateral plates of mesoderm,” al- 
though many sections give this impression. During this early 
period of development, when so many organs are rapidly being 
differentiated from embryonic tissue, it is impossible to tell 
exactly what forces or combination of forces are at work shift- 
ing the materials from one place to another. It is possible, as 
Allen suggests, that the germ-cells themselves take an active 
part in the processes which separate them from the endoderm, 
since it is apparently only through their own activity that 
they reach their final position in the embryo. Since Hertwig 
(41), Boveri (12), and others have traced the germ-cells 
back to segmentation stages, and Conklin (21, 22) has found 
various organ-forming substances in definite areas in the un- 
segmented egg, it seems meaningless to speak of organs as 
arising from any definite “germ-layer,” although the conven- 
ience of such a starting point for the study of the development 
of any structure is obvious. Owing to the character of the 
embryonic cells it is seemingly impossible to trace any organ 
in Bufo back to early cleavage stages, and the sex-cells are 
not clearly defined until the tadpole is about five days old. 
At this stage of development the germ-cells still retain their 
earlier embryonic character, and they are in contact with and 
closely resemble the endodermal cells. Instead of asserting 
that the germ-cells in Bufo are endodermal in origin, it seems 
to me more in keeping with the results of the investigations on 
other more favorable forms to assume that these cells in Bufo 
are of like generation with the primitive endodermal cells and 
that both kinds of cells arise from neighboring regions of 
the unsegmented egg. It may sometimes be possible to deter- 
mine the organ-forming regions in the unsegmented egg of 
Bufo as Conklin has done in the egg of Cynthia. 
