220 
largest occurring quite sparsely. The hypoblast of the area opaca is 
densely filled with much larger spherules, sometimes reaching a dia- 
meter of 27 u. At the boundary between these two areas, one finds 
a preponderance of spherules of intermediate size 5.5 «. The portion 
of this zone opposite to the posterior half of the embryo is destined 
to give rise to the sex-cells, although the latter can not be clearly 
distinguished as such until in later stages. The region of the blasto- 
derm outside of the area pellucida has received considerable attention 
from many authors. The literature upon the subject is so extensive 
that the reader must be referred to the reviews of it found in BRoNN’s 
Thierreich, Bd. 6, Abt. 3, No. 3 and in Herrwıc’s “Text Book of 
Embryology of Man Mammals”. In the earliest stages studied, this 
zone of sex-cells is clearly defined, its cells and those of the more 
peripheral hypoblast being sharply separated from the underlying yolk. 
This zone is wholly outside of the region involved in gastrulation. 
(See Mırsukurı ’93.) It seems possible, but hardly probable, that 
new “merocytes” may be from time to time added, up to slightly later 
stages of development. The cord-like anlage of the red blood cor- 
puscles appears immediately above the sex-cell zone. Unfortunately, 
sufficiently early stages were not at hand to determine whether or not 
the blood cells were derived from the hypoblast beneath (sex-cell an- 
lage), as asserted by a number of authors — RÜCKERT, KOLLMANN etc. 
From the insufficient evidence at hand, I am inclined to doubt it and 
to rather side with those who consider the slender blood anlagen to 
be anterior extensions of the mesoblast from the region near the 
blastopore. There certainly is no connection between these two anlagen 
after the egg is laid. 
With the formation of the mesoderm, the shape of the area pel- 
lucida becomes materially modified, being much narrower than in 
the earlier stages. This process brings the sex-cells nearer to the 
embryo. 
In the embryo of 1.7 mm total length, the sex-cells form loose 
clusters as seen in transverse section, and show an amoeboid form. 
They have begun to migrate between the other endoderm cells toward 
the median line in their journey to the sex gland anlagen. 
In the 2.8 mm embryo, the primitive sex-cells have become more 
scattered (Fig. 1). They seem to have lost their amoeboid form, in 
fact they are distinguishable from the other cells of the embryo by 
characters little in keeping with the idea of migration, namely, their 
large size, spherical form, and the presence of numerous yolk spherules 
of varying size. The latter serve to sharply differentiate them from 
