A TYPE OF EMBRYONIC DEVELOPMENT IN INSECTS 



21 



neuroblasts (Fig. 16, neur). The daughter cells of the neuro- 

 blasts nearest the neural groove develop into the median nerve 

 strand (Fig. 22, mst). The number of neuroblasts in any section is fairly 

 constant for a given species, most numerous in the region of the head 

 lobes. Active proliferation of the neuroblasts gives rise to numerous 



C D 



Fig. 24. — Blastokinesis. Successive stages A-D. {am) Amnion, {do) Secondary dorsal 

 organ, (fu) Fusion area of amnion and serosa (ser). 



daughter cells (nerve cells), which in cross section appear as a column 

 of cells with a neuroblast {neur) at the base of each. Ganglia are formed 

 in the body segments, and later the neural layer of cells separates from 

 the body wall. The neuropile (Punctsubstanz, fibrillar substance, nerve 

 fibers) develops in the ganglia in the older stages, at first free dorsally 

 but later surrounded by nerve cells. Commissures appear before the 

 connectives. In young embryos 6 pairs of ganglia belong to the head, 

 3 to the thorax, and 11 (more rarely 10 or 12) to the abdomen. In the 



