4 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. y-] 



sues of the future ventral nervous system. By multiplication, these 

 cells form a lateral cord of cells in each ridge, and a median cord 

 above the groove. According to Wheeler (1891), there are in insects 

 typically, as exemplified by the Orthoptera, four rows of primitive 

 neuroblasts in each ridge, and a median rovvr above the neural groove. 

 The neuroblasts divide repeatedly, those of the ridges giving rise 

 to the vertical columns of ganglion cells (fig. i B, GngCls) that 



Nbl 



Ect 



Gn^Cls g Nbl 



Fig. I. — Diagrammatic cross-sections of median ventral part of insect 

 embryo, showing origin of ventral nervous sj'stem. 



A, formation of neuroblasts (Nbl) from ectoderm (Ect) of neural 

 ridges (NIR) and neural groove (NIG). B, later stage (diagram 

 based on drawing by Wheeler, 1891) showing formation of median 

 cord (MC) and lateral cords of ganglion cells (GngCls) formed from 

 the original neuroblasts (Nbl). 



form the lateral cords, those above the groove producing a strand 

 of cells which is the median cord {MC). From these cells are later 

 formed the cellular and fibrous tissues of the ventral nerve cord. 



The ganglia of the stomatogastric nervous system are developed 

 from the dorsal wall of the stomodeum, which is of ectodermal 

 origin, but the details of their formation and separation from the 

 epithelium have not been as fully described as in the ventral ganglia. 



An animal with its nervous centers once buried within its body 

 must establish connections between these centers and its own exterior. 



