ORTHOPTEROIDEA (PANORTHOPTERA) 



223 



[leriphery. At about 10 hours, after several divisions, the eggs being 

 maintained at a constant temperature of 33°C., the cleavage cells start 

 migration to the periphery, which they reach by the 18-hour stage. 

 Meanwhile with repeated mitotic divisions there is an anterior migration 

 along the periphery of the egg but not through the yolk. About the 

 twenty-third hour the cells at the posterior end of the egg form a continu- 

 ous layer (Fig. 137) of primary epithelium (blastoderm). Anteriorly the 

 cells are not contiguous until about the twenty-eighth hour (Figs. 138, 

 139). The cells of the germ disk, or embryonic region (emb), are closely 

 packed together and columnar. Those of the extraembryonic region are 

 elongated tangentially to the egg periphery. 



Fig. 138. — Locusta. Cross section near posterior end. 28-hour stage, (emb) Embryonic 

 portion of blastoderm, {y) Yolk, (yc) Secondary yolk cell. 



The cells of the germ-disk epithelium undergo rapid division at about 

 the thirtieth hour, with the result that the epithelium temporarily acquires 

 an irregular two- or even three-layered arrangement of the nuclei in 

 places (Fig. 140). At this time the mid-ventral line of the germ disk near 

 its future cephalic end, a shallow groove, is formed on the outer surface. 

 This groove, which lasts for about four hours or less, is called the "first 

 ventral groove" and, according to Roonwal, is not to be confused with 

 the gastral groove. 



The yolk cells which lie immediately beneath the germ disk at the 

 thirtieth hour are arranged in a layer (Fig. 140, ycm), forming the yolk- 

 cell membrane. The cell boundaries are not visible, the nuclei being 

 connected by protoplasmic strands forming a continuous membrane 



