No. 2.] BLATTA AND DORYPHORA. 359 
THE FATE OF THE DIFFERENT GERM LAYERS. 
A. Entoderm. 
Recent writers on insect embryology have recognized two 
forms of entoderm,—one called primary, and represented by 
the yolk cells; and one secondary, represented by the’ epithe- 
lial wall of the mesenteron. In A/atza the yolk nuclei steadily 
increase in size from their first appearance at a time when they 
are no larger than other nuclei. At the time of the formation 
of the dorsal organ they are by far the largest nuclei in the egg 
(Fig. 51 v). Their chromatin is distributed through the karyo- 
plasm in the form of a fine, convoluted thread, and as two 
or more nucleoli (Fig. 54 v). Soon after the closing-in of the 
yolk, they lose their rounded outline, and become irregular and 
more homogeneous (Fig. 55 v). In the last stages of their 
dissolution they may be seen as stellate spots in the remains of 
the yolk aggregated in the stomach of the advanced embryo 
(Fig. 48). Yolk segmentation, though occurring in A/azza, takes 
place after the appendages are formed, at a much later period 
than in Doryphora. The segments, usually very obscurely de- 
fined, become confluent again as development continues. 
In Doryphora the yoke cells undergo no increase in size from 
the time of their first appearance ; but soon after the yolk has 
become segmented their cytoplasm is reduced to a scarcely 
perceptible layer surrounding the nucleus, which has become 
irregularly polygonal (Fig. 83). As all the eggs I studied were 
killed and preserved in the same manner, this difference in 
form between the yolk nuclei in the stages during yolk seg- 
mentation and after this process till the setting-in of degenera- 
tion, must be regarded either as a normal change in the living 
nuclei, or as indicating that their chemical nature is changed, 
and their resistance to the altering effects of reagents lessened. 
After the completion of the mesenteron at a time when the 
larva is almost ready to hatch, the remains of the yolk nuclei 
are pushed back into the stomach in the same way as in Blatza. 
Here they degenerate in a manner very similar to that observed 
in the secondary entoderm nuclei, and the nuclei of the amniotic 
dorsal organ. They become swollen and vesicular, and their 
chromatin is reduced to irregular masses. The yolk becomes a 
compact, granular mass, staining pink in borax carmine. 
