No. 2.] BLATTA AND DORYPHORA. 331 
resenting cross-sections through the middle of the caudal plate. 
In Fig. 87, from a younger embryo, the gastrula has not yet 
closed. Its walls are seen to be much thickened, and the kary- 
okinetic figures. show that its component cells are still prolifer- 
ating. At the lower surface of the bag-shaped mass the cells 
are somewhat less compact and form a layer (ez¢) which at some 
points is separated from the superjacent cell-mass. This mass 
will give rise to the entoderm, and that above it to the meso- 
derm, as soon as the orifice x is definitely closed. [Ve thus havea 
mass of cells in which all three germ layers blend, and to no part 
of which can be assigned the name of a germ layer. Not till the 
groove ts closed have we mesoderm, and not till the lower cells of 
the mass have become clearly differentiated from those above them 
can we speak of entoderm. 
The section Fig. 88 is from an embryo in which three germ 
layers are definitely formed, shortly after the closing of the gas- 
trula. A depression (7) marks the point where the proctodzal 
invagination is to occur. The polygonal mesoderm cells are 
spread out in a mass (sd), which is separated by a more or less 
distinct line (/) from the entoderm beneath (ext). The differ- 
ences between the cells of the last layer and the superjacent 
mesoderm are difficult to represent. Their nuclei are somewhat 
larger and clearer. They gradually merge into the mesoderm 
cells, the boundary being exceptionally clear in the section fig- 
ured. Heider (19) says of these same entoderm cells in Hydro- 
philus that they are more “succulent” than the mesodermic 
elements. This adjective conveys the idea more clearly than 
paragraphs of description. 
The peculiar nuclei which under a low power seemed to be 
dividing are now seen to be ina process of dissolution. TZhey 
originate in the entodermtic mass and pass into the adjacent yolk, 
where they disappear, sections through slightly later stages 
showing no traces of them. From what I have seen I believe 
these nuclei to pass through the following stages, examples of 
all of which may be found in a single embryo. The karyochy- 
lema becomes vacuolated, probably with substances absorbed 
from without, to judge from the large size of some of these 
nuclei (Fig. 88 v), while the chromatin ceases to present the 
threadlike coil and becomes compacted into irregular masses be- 
tween the vacuoles. Finally, the vacuoles fuse and the masses of 
