SEX-CELLS OF AMIA AND LEPIDOSTEUS 27 
cell anlagen are pushed together in the median line, between the 
aorta and the roof of the archenteron. They fuse into a median 
longitudinal rod of cells lymg just above the dorsal root of the 
mesentery. By this time the sex-cells have lost their yolk 
material and have, to a large extent, assumed their definitive 
character. During these stages the number of the sex-cells has 
increased from one hundred to one hundred and fifty, occasional 
mitoses being observed. Soon after this stage of the median 
anlage (9 mm.) has been reached, the sex-cells migrate laterally 
to their final positions on each side of the root of the mesentery. 
At the stage of 14 mm., a large number of them degenerate, leav- 
ing only 60. A second generation of sex-cells soon begins to form 
from a source entirely different from the first, namely, from a 
transformation of ordinary peritoneal cells. Dustin is, in this 
regard, quite in accord with Bouin who expressed similar views 
regarding Rana. Dustin considers somewhat more briefly the 
corresponding stages in Rana and Bufo. Here he finds what he 
considers to be a substantially similar source of origin of the sex- 
cells, namely the medial borders of the lateral plates of meso- 
derm. An incredible feature of his account is the statement that 
the lateral sex-gland anlagen contain no sex-cell at all comparable 
in size to those of the yolk-filled entoderm, at the period imme- 
diately prior to their union in the median line. Dustin would 
have us believe, nevertheless, that these selfsame sex-cells show 
a close resembiance to the entoderm cells immediately after this 
union of the lateral anlagen, and this in spite of the fact that both 
of these stages of development are so close together that the 
embryos upon which he made these observations were all of the 
same length. His own statement is as follows: 
‘“Au moment ot les ébauches paires séparées par une sorte de clivage 
des lames latérales du mésoblaste se sont rapprochées de la ligne médiane, 
les cellules sexuelles futures passent par une série de transformations 
cytologiques a la suite desquelles elles auront presque les caractéres des 
cellules de l’hypoblaste vitellin. Les dimensions des corps cellulaires 
augmentent dans de fortes proportions; les grains vitellins deviennent 
beaucoup plus nombreux et plus volumineux; ils se colorent mieux par 
orange G. Par le fait de l’augmentation du nombre des plaquettes 
