No. 2.] BLATTA AND DORYVPHORA. 363 
entoderm have reached the middle of the embryo a number of 
granular and somewhat larger cells are to be found mingled with 
the cells of the bands. I conclude that these cells are of ento- 
dermic origin because when first seen they are associated with 
the entoderm cells and resemble them more closely than they 
resemble the adjacent mesodermic elements. At first small 
(Fig. 85 ad), these fat cells gradually but constantly increase in 
size, their cytoplasm and nuclei increasing in about the same 
ratio. They wander about in the body cavity, but finally attach 
themselves to the ectodermic body walls, especially in the pos- 
terior two-thirds of the embryo on each side of the heart (Fig. 
86 ad). They remain more or less globular or oval, the side in 
contact with the wall hollowing out a concavity in the cells of 
the ectoderm. The granulation of the cytoplasm which first 
distinguishes the fat cells from the true entodermic elements 
becomes coarser with the increase in volume. In the embryo 
ready to hatch the adipose cells have acquired gigantic dimen- 
sions, being many times the size of those represented in Fig. 86. 
Both nuclei and cytoplasm stain deeply, so that these fat cells 
are rendered among the most conspicuous objects in a section. 
B. Mesoderm. 
In Doryphora as soon as the gastrular tube has collapsed, the 
polygonal mesodermic elements form a layer several cells in 
thickness, applied to the inner surface of the median ventral 
ectoderm (Fig. 65 msd). This layer of cells thins out at its 
lateral edges. With the first traces of segmentation in the outer 
layer the mesodermic layer also divides, though incompletely, at 
the same places of constriction (Fig. 82). Soon the single 
intersegmentally divided band of mesoderm splits in the median 
line so that each segment contains two subquadrangular flat- 
tened masses. The ccelomic cavity is formed at the outer 
edge of each of the masses by a separation of the cells of the 
two layers (Fig. 78 c/). The inner constitutes the splanchnic 
mesoderm, while all the remainder of the mesoderm constitutes 
the somatic layer (Fig. 78 s/m). When the appendages appear, 
it is the latter layer of cells which supplies their cavities with 
muscle-forming cells; the portions inclosing the ccelomic cavity 
accompany the adjacent ectoderm in its dorsad growth. As 
