32 



ECONOMIC ENTOMOLOGY 



the endoblast (meso-entoderm) . This stage of the embryo is known as 

 the gastrula. 



Meanwhile the blastoderm is folding over the germ band from 

 either side, producing an inner membrane — the amnion, and an outer 

 membrane — the serosa (Fig. 35). 



Two types of germ bands may be noted. The overgrown type 

 retains its original position (Fig. 36), and the blastoderm folds over 

 the germ band from either side forming the two layers amnion and 

 serosa. In the invaginated type, seen in aphids and Odonata (Fig. 37), 

 the germ band invaginates into the egg so that its ventral surface 



faces the dorsal surface of the egg. 

 At a later stage the embryo turns 

 and regains its original position. 



The germ band shows early 

 signs of segmentation, beginning 

 first at the anterior end. An in- 

 vagination of the ectoderm near 

 the anterior end forms the stomo- 

 doeum or fore gut, and a similar 

 posterior invagination forms the 

 proctodceum or hind gut. The 

 segmentation shows about 21 seg- 

 ments, 6-7 to the head, 3 to the 

 thorax, and 11-12 to the abdo- 

 men. On each segment except 

 the first and last a pair of tiny limbs (buds) are seen. 



Soon the germ band widens and closes over the yolk to form the 

 dorsal waU. 



Before dorsal closure occurs the beginnings of the nerve-cord 

 form in the median groove. This primitive nerve-cord is double, 

 and a pair of swellings in each segment develop into the ganglia of 

 the ventral system. 



The tracheas arise as invaginations of the ectoderm. 

 The entoderm arises from the endoblast, the inner embryonic layer, 

 as two cell masses situated at either end of the embryo. These masses 

 grow backward and forward respectively, and unite to form the mid- 

 gut {mesenteron) . 



The rest of the inner layer forms the mesoderm from which arises 



Pig. 36. — Formation of the embryonic 

 membranes — overgrown type. Am., am- 

 nion; Ect., ectoderm; Enl., entoderm; 

 Ser., serosa. {After Korschelt and H eider.) 



