300 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



Donacia (Hirschler, 1909) and Euryope (Paterson, 1932), the primary 

 dorsal organ lasts but a short time. It appears when the blastoderm is 

 complete at the thirtieth hour but disappears before the ventral plate has 

 become enclosed by the amniotic folds. 



The thickening of the posteroventral blastoderm to form the germ 

 band occurs about the thirty-fifth hour. The amniotic folds develop 

 from the edges of the germ band and eventually fuse along the middle line 

 to enclose the germ band with two envelopes, the amnion on the ventral 



Fig. 242. — Brachyrhinus. Longitudinal section, (am) Amnion, {ect) Ectoderm. {U) 

 Inner layer, {ser) Serosa, {sub. aer) Subserosa. 



side, and the serosa enclosing the contents of the egg. The germ band 

 now invaginates into the yolk, moving inward from the posterior pole tail 

 first until only the head lobes remain at the surface (Fig. 242). 



The embiyo now begins to lengthen and at the same time to withdraw 

 from the yolk. It withdraws in a lateral direction so that if the egg is 

 viewed from one side, the embryo appears somewhat S shaped with the 

 tail still immersed in the yolk. When the withdrawal process is complete, 

 the tail lies diagonally over the side of the yolk while the head and the 

 fore part of the embryo still lie in a vertical plane at the posterior pole of 



