= nr 
221 
the surrounding cells of the area pellucida endoderm, because the yolk 
material has long since disappeared from the latter. See Fig. 5. 
It is not our purpose to enter into a consideration of cytological 
details, but a few points might be noted in passing. During these 
early stages the nuclei of the sex-cells not only show a striking simi- 
larity to those of the surrounding endoderm cells, but to those of the 
other germ-layers as well. Fig. 5. The nuclei of the sex-cells are 
spherical, while those of the general endoderm cells have a more or 
less oval form. The chromatin appears in the form of minute granules 
strung upon a linin network. There is almost invariably but a single 
nucleolus which lies in or near the center of the nucleus. 
The sex-cells are most numerous opposite the posterior portion 
of the embryo, but throughout this and the succeeding stages they 
are found to occur as far forward as a point opposite the anterior 
portion of the pronephros (see 
Fig. 1) although they are scarce in 
these anterior regions. 
At this stage there are, in 
all, 13 mesoblastic somites. Since 
the anterior end of the sex gland  Pro-neph. 
eventually appears immediately en 
behind the 15th. somite one can FR 
readily see that it will be formed 
wholly in the region of the em- 
bryo which shows no evidence of R fe | 
segmentation at this period. It oneal 
is clear that the zones of sex- a 
{ cells here represented are far Fig. 1. Embryo of 2,8 mm total length. 
- = . Ventral view. The black dots near the peri- 
more extensive in an antero- phery of the area pellucida indicate the 
posterior direction than is the positions of sex-cells determined From a study 
region in which the sex gland ee transverse sections. Pro-neph. 
will arise. It would be of great 
interest to know whether or not the sex-cells follow converging paths 
towards the sex gland region, and if so, to what extent. In later stages, . 
no sex-cells are to be found posterior to the embryo. This may be 
due to a folding forward of the endoderm of that region to assist in 
the closure of the alimentary tract, coupled no doubt with independent 
migration of the sex-cells. In any case the sex-cells are extremely 
numerous in the posterior part of the embryo towards the end of mi- 
gration. It may be possible that at least some of these sex-cells may 
degenerate in situ or be transformed into somatic cells, although I 
