TRICHOPTERA AND LEPIDOPTERA 341 



at intervals swollen places which mark the position of the nuclei. After 

 completion of the serosa, the edge of the germ band becomes slightly 

 reflexed; the edge later (Figs. 298, 299^) thins out, expands, and finally, 

 at 28 to 30 hours, completely covers the whole ventral face of the embryo 

 as a very thin membranous amnion marked here and there with swollen 

 places which indicate the position of the nuclei. 



While the amnion is forming, the embryonic rudiment increases in 

 size through active mitosis, forming a cup-like disk with inturned edges 

 (Figs. 297^1, -B) penetrating the yolk. The cephalic and caudal ends of 

 the rudiment close over the dorsal (yolk) side, forming an anterior 



Vsp 



emb 



Fig. 298. — Diacrisia. 20-hour embryo, {am) Amnion, (emb) Embryo, (ser) Serosa. 

 (y) Yolk, (ysp) Yolk spheriile. 



two-lobed and a posterior simple pouch. About this time the yolk 

 material lying immediately beneath the surface forms yolk spherules, 

 each enclosing one or, less commonly, two or more yolk cells (Fig. 298). 

 In the course of the next few hours the entire yolk is reduced to large 

 yolk spherules, each with one or more yolk cells. The nuclei of these 

 cells are distinctly larger than those of the germ band though smaller 

 than the nuclei of the serosa at this time. Each spherule is surrounded 

 by a delicate yolk membrane. The cephalic lobes are now quite distinct, 

 the body acquiring in the course of the next 24 hours its greatest length 

 (Fig. 297Z)). While the germ disk is elongating, a median strip 8 to 10 

 cells in width (Figs. 299B,C) sinks into it. This strip extends from the 

 head lobe longitudinally to the tail, though at first it is not in evidence 

 in either the head or the tail pouch, nor does it appear uniformly and 

 simultaneously throughout its length. The growth of the lateral parts 

 of the ectoderm toward the median line and their final fusion result in 

 the median strip's becoming the inner layer (Figs. 299C,Z)). The sinking 



