SIPHONAPTERA AND DIPTERA 



363 



migration, the inner walls of the blastoderm cells form, and the blastoderm 

 is complete (Fig. 316). 



Between the twelfth and the thirteenth hours the ventral wall of the 

 blastoderm thickens, forming the ventral plate. A groove now appears 

 on the ventral side, marking off a triangular section. The broader side 

 of the triangle lies toward the anterior pole of the egg; the blunt apex 

 reaches nearly to the posterior end. The triangle will form the embiyo, 

 the broad end becoming the head, the apex forming the tail. 



Fig. 316. — Sciara. Longitudinal sec- 

 tion of 12-hour blastoderm {bid), igc) 

 Germ cells, (y) Yolk cells. 



Fig. 317. — Sciara. Longitudinal 

 tion at 15 hours, {am) Amnion, {am. 

 cav) Amniotic cavity, {do) Primary dorsal 

 organ, {gb) Germ band, {gc) Germ cells. 

 {ser) Serosa. 



In a live egg the transverse crease marking the anterior limit of the 

 future embryo appears first. Within fifteen minutes the blastodermal 

 cells lying anterior to the crease fold over the embryo and grow toward 

 the posterior end of the egg. They thus form the anterior amniotic 

 fold. Although this fold can at first be seen only in the anterior end, 

 it is growing at the same time over the lateral edges of the embryo which 

 is invaginating into the yolk (Fig. 318). The fold forms last of all over 

 the posterior extremity. The area of the embryo left uncovered by the 

 amniotic fold on the ventral side (Fig. 317) constantly grows smaller 

 until by the fifteenth hour the embryo is covered over entirely and lies 

 inside, protected by the two envelopes. 



