336 WHEELER. [Vot. III. 
through the point ¢ of Fig. 43, a region at which the two folds 
have not yet arisen. The ectoderm (ecd) is very thick, espe- 
cially in the median ventral line. The mesoderm (msd@) is 
incompletely separated from the ectoderm, and is seen only in 
the median portion of the section. The yolk cells are large and 
distinct. Their chromatin forms large nucleoli. The yolk under 
the embryo is becoming much vacuolated. 
The section (Fig. 32) passes through a point a little in front of 
as in Fig. 43. This embryo was older than the one represented 
in Fig. 42; hence the yolk cell (v) is much larger and the meso- 
derm more distinctly separated from the ectoderm. The infold- 
ing of the ectoderm to form the amnion and serosa is seen on 
either side. The amnion (am) is thicker than the serosa (s7). 
Its nuclei are small, close together, and spherical, while the 
nuclei of the serosa are large, flat, and scattered. Figure 301s a 
section through a point near e! (Fig. 43), of a slightly older em- 
bryo. The amnion and serosa are completed and in close con- 
tact with each other, so that the yolk cannot pass in between 
them. The difference between the nuclei of the amnion and 
serosa is very pronounced. The mesoderm (sd), which is 
several cells deep, and is now distinctly separated from the 
ectoderm, has become an independent layer. Its nuclei are 
more spherical than the ectoderm nuclei, many of which are 
considerably elongated and flattened. 
The entoderm is formed very late in Alatta (about the 15th 
or 16th day), and in cross-sections of embryos of that age it 
may be seen as a thin layer of cells on either side of the ventral 
yolk closely applied to the inner face of the splanchnic meso- 
derm (Fig. 54 ext). Cholodkovsky (10) claims that these bands 
of entoderm are derived from the splanchnic mesoderm by de- 
lamination, but an examination of a number of sections has 
convinced me that it is next to impossible to come to any defi- 
nite conclusion as to the mode of origin of the entoderm in 
Blatta. By the time it has begun to form the cells have become 
very minute, and different tissues of very different origins have 
become closely united, so that the proximity of splanchnic meso- 
derm and entoderm is no proof of the derivation of the latter 
from the former. I have been able to satisfy myself, however, 
that the entoderm appears in two thin layers, one on each side 
of the median ventral yolk, and that these two layers converge 
