98 



EMBRYOLOGY OF INSECTS AND MYRIAPODS 



constricted into segmental divisions. At the time the yolk recedes from 

 the germ band, nerve fibers begin to develop on the ganglion cells that 

 he on the dorsal side of the lateral cords. These ganglion cells lengthen, 

 the inner ends of each being produced into a plasmic process which elon- 

 gates and probably in most cases branches, interlacing with the plasmic 



am 



neurp mst 



neur 



Fig. 45. — Xiphidium. Cross section of an early stage, {am) Base of amnion, {ect) 

 Dermatogene ectoderm, {neur) Neuroblast, (neurg) Neural groove. {From Wheeler.) 



fibers of other cells to form the neiiropile, or Punctsubstanz, of the ganglia 

 (Fig. 46, neury). This process of fiber formation proceeds from before 

 backward. Meanwhile the cells that lie more deeply in the lateral cords 

 also develop similar nerve fibers. 



The metameric division of the lateral nerve cords at the time of body 

 segmentation results in the formation of nodes of ganglionic cells on each 



cord. The nodes of one cord, fusing 

 with the corresponding nodes of the 

 other, form the ganglia of the central 

 nervous system. Certain nerve cells 

 in the ganglionic node of each cord 

 send fibers across to the correspond- 

 ing node on the other side, giving rise 

 to two commissures. Other cells 

 likewise send fibers into the preced- 

 ing and following nodes to form the 

 pair of connectives. Fibers in the 

 commissures are in some insects also 

 in part derived from the ganglion 

 cells of the median cord. Although 

 there is a close fusion of the gan- 

 ghonic nodes to produce the defini- 

 tive ganglia of the ventral nerve cord, thereby obliterating external 

 evidence of the commissures, the connectives usually remain separated, 

 retaining their individuality. For a time the neurogene cells remain in 

 close contact with the dermatogene (ectoderm) cells, but later, as develop- 

 ment proceeds, the nerve cord separates from the adjacent tissue and 

 becomes surrounded by the neurilemma, a very thin covering which here 



neur 



A 



neurg 



Fig. 46. — Section of ventral nerve 

 cord, {dt) Daughter cells, (eci) Ecto- 

 derm, {mst) Median nerve strand. 

 {neur) Neuroblast, {neurg) Neural 

 groove, {neurp) N e u r o p i 1 e. {From 

 ler.) 



