No. 2.] BLATTA AND DORYPHORA. 333 
Blatta. 
My observations on the formation of the germ layers in Blatta 
are less satisfactory than those on the same process in Doryphora, 
because the eggs of the former are difficult to section and have 
small, indistinct cells in the later stages. I have, however, 
given much attention to the subject, sufficient, I believe, to be 
able to assert that the method of germ layer formation departs 
from the type observed in Hydrophilus and Doryiphora. 
As soon as the blastoderm is completed by the rapid prolifer- 
ation of the blastema cells, the whole layer of protoplasm 
with its embedded nuclei contracts from the lateral faces 
towards the front of the egg. The blastoderm thus becomes 
exceedingly thin on the lateral and dorsal surfaces of the egg, 
and the nuclei of these surfaces become much scattered and 
flattened, while the protoplasm is thickened on the whole ven- 
tral face, where the nuclei are crowded together and have again 
become spherical. A slight further contraction away from the 
cephalic and caudal ends towards the centre of the ventral face 
shortens this mass of thickened cells into the ventral plate. 
While the blastoderm is thickening, nuclei are being given 
off centripetally to form the yolk cells. A few of these nuclei 
go deep into the yolk, but the great majority remain at or very 
near the surface. They are not given off in a continuous sheet, 
nor are they produced from the blastoderm by any invagination. 
They are simply nuclei which have been sent into the yolk from 
different and often widely separated points of the contracting 
blastoderm. The few nuclei which descend into the yolk 
remain for a long time small and indifferent. Sometimes the 
number of these nuclei is very limited so that they occur in 
only a few of a great number of complete longitudinal sections 
passed through an egg. 
The nuclei of the surface yolk undergo considerable differen- 
tiation, and are soon easily distinguished from the superjacent 
blastoderm. They surround themselves with stellate cytoplasm, 
retain their spherical or spheroidal shape, and often present one 
or more large nucleoli (Figs. 29 and 32 v)._ Their function for 
many days is the conversion of the yolk into soluble compounds 
to be absorbed by the rapidly dividing cells of the embryo. 
During this process they grow rapidly, and soon become the 
