326 WHEELER. [Vor. III. 
THE FORMATION OF THE GERM LAYERS AND EMBRYONIC 
ENVELOPES. 
Doryphora. 
As my observations on the formation of the germ layers in the 
potato-beetle are both more copious and more satisfactory than 
in Blatta, and as Doryphora probably represents more nearly 
the typical process of germ layer formation, I will begin my 
remarks on this subject with Doryphora, and append what I 
have to say on Liatta. 
The first change visible in the blastoderm from the surface is 
the appearance of a pair of folds which arise on the middle of 
what is to be the ventral surface of the egg (Fig. 66). The two 
folds may best be described as resembling a pair of but slightly 
bent parentheses close together. In cross-sections these folds 
are scarcely perceptible, and the difference in thickness between 
the ventral and dorsal blastoderm is very slight. 
Soon the folds became more decided (Figs. 67 and 68), their 
anterior ends are continued around and meet so as to inclose 
a spade-shaped space, while their posterior ends diverge and are 
continued in the opposite direction to the caudal end of the egg, 
where the depression inclosed between the two folds turns 
inward and ends abruptly. The depression inclosed by the 
fold is to become the groove-shaped gastrula. The nuclei of 
the blastoderm now present a very different appearance from 
that represented in Fig. 66. The fold-surrounded depression 
at the anterior end (Fig. 67 a), lying where the stomodeum 
is subsequently invaginated, is apparently a differentiated por- 
tion of the gastrula, to judge from its peculiar shape. In 
lateral view (Fig. 68 a) this portion of the blastoderm is seen 
to be concave, the nuclei are closely aggregated, whereas an- 
teriorly and posteriorly they are much scattered and have 
increased in size. The true distribution of the cells, as shown 
by their nuclei, is best seen in the lateral view of the same 
egg (Fig. 68). The aggregated mass of cells, or the ventral 
plate as it may now be called, is clearly marked off from the 
serosa or remaining blastoderm, which is recognized by its 
larger and more scattered nuclei. The ventral plate is seen 
to be constricted toward the middle of the egg to form two 
