No. 2.] BLATTA AND DORVPHORA. 335 
tion through the point pel of the egg represented in Fig. 41. ecd 
is the thickened ectoderm, msd the mesoderm, which is absent 
in the middle of the ventral face at c. 
That the mesoderm grows forward from the rounded caudal 
thickening may be proved by sections through a number of 
eggs taken from capsules six and one-half to eight days old. 
The eggs will be found in various stages more or less close to 
that given in Fig. 41. In some a very short row of mesoderm 
cells is found just in front of the thickening ; in others the row 
will be longer as it has advanced further to the cephalic end. 
Only part of the mesoderm is formed at the thickening. As 
can be concluded from the even arrangement of the two layers 
anterior to dp in Fig. 38, each ectoderm cell has a mesoderm 
cell beneath it, showing that the mesoderm is derived from the 
ectoderm by centripetal division. The impulse to this division, 
however, seems to originate in the incomplete invagination at 
bp and to travel towards the head of the embryo. 
After the mesoderm is formed, the depression 6f disappears, 
and the amnion and serosa begin to develop. They rise as a 
crescentic fold from the rounded posterior edge of the area of 
proliferation (Fig. 42 as). The cells of the procephalic lobes 
become more prominent, and while the caudal fold of the 
amnion and serosa is growing in length and continuing up the 
edge of the ventral plate, a fold also arises from the outer edge 
of each procephalic area and bends inward. This stage in the 
development of these membranes is seen in Fig. 43. 
The embryo is now slipper-shaped, the toe of the slipper 
being the caudal and the heel the cephalic end. The growth of 
the membranes continues, the toe of the slipper completing 
itself more rapidly than the heel. Soon the two procephalic 
folds are connected around the anterior tip of the ventral plate, 
which is undergoing a change in outline. Figure 44 shows the 
amnion and serosa almost closed. Over the spot where the 
stomodzeal invagination will soon appear, there is still a small, 
slit-shaped opening in the membranes, but this soon closes; 
not, however, till after the appendages, both cephalic and tho- 
racic, have begun to appear. 
The structure and formation of the amnion and serosa, as well 
as their relations to each other, can be made out from the sec- 
tions (Figs. 29, 30, and 32). Figure 29 is a transverse section 
