ENTOMOLOGY 



and are therefore ectodermal in origin. This primitive double nerve 

 cord becomes constricted at intervals into segments, or neuromeres, 

 which correspond to the segments of the germ band. Each neuromere 



FIG. 200. Diagrammatic sagittal sections to illustrate invagination of germ band in 

 Calopteryx. a, anterior pole; ac, amnion cavity; am, amnion; b, blastoderm; d, dorsal; 

 g, germ band; h, head end of germ band; p, posterior pole; s, serosa; v, ventral; y, yolk. 

 After BRANDT. 



PIG. 201. Diagrammatic sagittal sections to illustrate revolution of Calopteryx 

 embryo, a, antenna; am, amnion; I, labium; l l ~l 3 , thoracic legs; m, mandible; mx, maxilla; 

 5, serosa. After BRANDT. 



consists of a pair of primitive ganglia, and these are connected together 

 by paired nerve cords, which later may or may not unite into single 

 cords; moreover, some of the ganglia finally unite to form compound 

 ganglia, such as the brain and the subcesophageal ganglion. In front of 



