HYMENOPTERA 



325 



this dorsal strip become aggregated in the cephalodorsal region of the 

 egg to form a more or less discoidal mass, the "cephalodorsal body." 

 This body appears to be nothing more than a thickened anterior end of 

 the dorsal strip. It later degenerates in the yolk, 

 do 



neurg 



Fig. 281. — Apis. Cross sections of successive stages, (do) Primary dorsal organ. 

 iect) Ectoderm, {it) Inner layer, {mge) Mid-gut epithelium, {neurg) Neural groove. 

 iser) Serosa. {In part from Snodgrass.) 



Ectodermal tissue is formed from the lateral plates, mesoderm from 

 the middle plate, and the mid-gut rudiments are interpreted by some 

 writers as the entoderm. During the formation of the germ layers both 

 the middle and the lateral plates show distinct evidence of body seg- 

 mentation which appears to correspond to the definitive segmentation 

 of the embryo. 



The serosa is single-layered and is formed from the dorsal half of the 

 blastoderm. It separates from the dorsal margins (Fig. 281 A, ser) of 

 the lateral plates and gradually grows over the ventral face (Fig. 281 B,ser) . 



