A TYPE OF EMBRYONIC DEVELOPMENT IN INSECTS 



13 



amf 



end pushes into the yolk carrying with it the adjacent primary epitheUum. 

 A pocket is thus formed at the posterior end of the egg, the inner wall of 

 which is the tail end of the germ band, the outer wall the amnion, and 

 the invagination itself the am- 

 niotic cavity (Fig. 11, am.cav). 

 A fold next appears on each side 

 which later extends to the head 

 end. These amniotic folds {amf) 

 including the tail fold increase in 

 width, the edges approaching each 

 other (Fig. 12) until they meet, 

 thereby closing over the germ 

 band. The outer surface, com- 

 posed of pavement cells, constitutes 

 the serosa {ser), the cover over the 

 ventral face of the germ band (em- 

 l)ryo rudiment), the amnion, the 

 space between amnion and germ 

 band, the amniotic cavity {am.cav). 

 The amnion, which is regarded as a 

 derivative of the germ band and has 

 a similar structure, later becomes 

 membranous (Figs. 13, 14). As the 

 tail of the germ band invaginates, 

 it carries with it the germ cells. 



Gastrulation and Formation of the Inner Layer. — While the embryonic 

 envelopes are forming, gastrulation of the germ band takes place. A 

 furrow-like invagination forms which begins anteriorly at the point where 



X. il 



am.cav 



Fig. 12. — Sagittal section of germ 

 band, {am) Amnion, (am.cav) Amniotic 

 cavity, (amf) Amniotic folds, (gc) Germ 

 cells, (ser) 



gastr 



Fig. 13. — Gastnilation. Cross section of germ band, (am) Amnion, (ect) Ectoderm. 

 (gastr) Gastriila fvuTOW. (il) Tubular inner layer, (ser) Serosa, (yc) Yolk cells. 



the stomodaeal invagination will arise and extends posteriorly to the 

 caudal end of the germ band, or "embryo," as we shall now term it. The 

 invagination does not form in its entire extent at once, and it may in some 



