38 Development and Metamorphosis 
imbedding and section-cutting which are not often found outside the college 
laboratory. But the particularly interesting and suggestive stages in this 
development may be outlined and illustrated in brief space. First, the 
germinal cell near the center of the egg divides repeatedly (Fig. 68 A) and 
the resulting new cells migrate outward against the inner envelope of the 
egg and arrange themselves here in a single peripheral layer, called the 
blastoderm (Fig. 68 D , bl). On what is going to be the ventral side of the 
egg the cells of the blastoderm begin to divide and maSs themselves to form 
the ventral plate (Fig. 69 C). The embryo is forming here; the rest of the 
blastoderm becomes modified and folded to serve as a double membranous 
envelope (called amnion and serosa) for the embryo. Stretching nearly from 
pole to pole as a narrow streak along the ventral aspect of the egg, the 
A B C o 
Fig. 68 .—Early stage in development of egg of water-scavenger beetle, Hydrophilus sp. 
A, first division of nucleus; B, migration of cleavage-cells outward; C, beginning 
of blastoderm; D, blastoderm; y., yolk; dc., cleavage-cells; yc., yolk-cells; hi., 
blastoderm. (After Heider; greatly magnified.) 
developing embryo begins soon to show that fundamental structural charac¬ 
teristic of insects, a segmental condition (Fig. 69 D). One can now make 
out the forming body-rings or segments, and each soon shows the beginnings 
or rudiments of a pair of appendages (Fig. 69 E). The appendages of the 
head and thoracic segments continue to develop and begin soon to assume 
their definitive character of antennae, mouth-parts, and legs, but those of the 
abdominal segments never get farther than a first appearance and indeed 
soon disappear. In the mean time the internal systems of organs are grad¬ 
ually developing, the ventral nerve-chain first, then the alimentary canal, 
and later the muscles, tracheae, and the heart. All the time the yolk is 
being gradually used up, fed on, by the cells of the developing and growing 
embryo, until finally comes the disappearance of all the stored food, and the 
time for hatching. 
