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No. 2.] BLATTA AND DORYPHORA. 361 
forward-growing strands, and terminating in the same acute 
point which seems to force its way between the yolk segments. 
These points of entoderm do not grow far, as I have concluded 
from an examination of slightly older stages, but soon fuse with 
the bases of the two main strands, and form a meniscoid mass 
in every way comparable with the watch-glass-shaped mass in 
Musca, as described by Kowalevsky (27). 
A median cross-section of an embryo, with the entoderm 
bands fully established, but not confluent, is shown in Fig. 78. 
The embryo is cut in two places. The upper half passes through 
one of the basal abdominal segments, while the lower half passes 
through the abdomen, a short distance from the tail. In the 
upper half no entoderm cells are to be found, as the two bands 
have not yet reached in their forward growth the basal abdominal 
segments in which they fuse with the two bands growing back 
from the stomodzal invagination. In the lower half a heap of 
succulent entoderm cells is seen on each side, separated from 
the coelomic cavity (c/) by the splanchnic mesoderm (s/). 
Four stages in the formation of the mesenteron after the 
establishment of the entoderm as two long bands, are repre- 
sented in Figs. 83 to 86 ext. The entoderm remains throughout 
embryonic development perfectly distinct from the splanchnic 
mesoderm, to which it is nevertheless very closely applied. At 
first the cells are irregularly arranged in the band which is 
deepest in the middle, but gradually flattens out to a single cell 
in thickness at its dorsal and ventral edges (Fig. 83). In a later 
stage, however, the nuclei have their long axes directed at right 
angles to the long axes of the splanchnic mesoderm cells (Fig. 
84), and thus indicate that the cells of the entoderm are begin- 
ning to assume a definite columnar arrangement, though they 
still lie, in some places, in two or more rows, one above the 
other. By the time the body walls are about to close, the cells 
of the entoderm have formed an even layer of columnar ele- 
ments (Fig. 85), an arrangement which is retained in all the 
subsequent stages till the embryo hatches. 
The growth of the entoderm, accompanying the adjacent 
mesoderm and ectoderm in their dorsad movement, is at first 
largely along the dorsal edges of the bands, as may be seen by 
comparing Figs. 84 and 85, where the distance between the 
ventral edges of the two bands is nearly the same, while the 
