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. 68/1) 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 (cdled amnion and serosa) for the embryo. Stretching nearly from 

 pole to pole as a narrow streak along the ventral aspect of the egg, the 



Fig. 68.— Early stage in development of egg of water-scavenger beetle, Tlydrophilus sp. 

 A, first division of nucleus; B, migration of clcavage<clls outward; C, beginning 

 of blastoderm; D, blastoderm; y., yolk; dc, cleavage-cells; yc, yolk-cclls; 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. tc/E). The appendages of the 

 head and thoracic segments continue to develop and begin soon to assume 

 their definitive character of antennte, 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, trachea?, 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. 



