THE NERVOUS SYSTEM. 



325 



An unusually regular arrangement of the cells is shown in sections 

 through the rudiments of the ventral cord in many Orthoptera (Fig. 

 159). Wheeler (No. 94) has recently recognised as "neuroblasts" 

 four large cells (n-n) lying on each side on the surface of the lateral 

 cords in Xiphidium; these cells, by repeated tangential division, 

 give rise to the ganglionic cells, which are consequently arranged 

 in vertical columns (.:). Graber (No. 30) and Viallastes (No. 84) 

 have observed similar phenom- 



n i n, 



Fio. 159. — Transverse section through the rudi- 

 ment of the ventral cord of Xiphidium (after 

 Wheeler). /, fibrous tissue in transverse sec- 

 tion ; m, neuroblast-cells of the median cord ; 

 n 1 -7i i , neuroblasts of the lateral cords; z, 

 column of ganglion-cells proceeding from the 

 neuroblasts. 



ena, the former in Stenobothrus 



and the latter in Mantis.* 

 The middle cord, according to 

 Wheeler, has neuroblasts (m) 

 only in the interganglionic 

 region ; these, however, soon 

 shift to the posterior side of 

 the transverse commissures. 

 In any case, as Wheeler has 

 pointed out, the presence of 

 eight longitudinal rows of 

 neuroblasts points back to a 

 similar condition in the Anne- 

 lida, where only two such rows, 

 produced from neuroblasts, are 

 found (Vol. i., p. 294). 



The rudiments of one pair of ganglia of the ventral chain originally 

 appear in each of the sixteen segments of the primary trunk. Fusion 

 may occur between these rudiments later, and may bring about an 

 apparent reduction in their number. The ganglia of the three 

 maxillary segments, for instance, unite to form the sub-oesophageal 

 ganglion, and the last pairs of abdominal ganglia fuse in varying 

 numbers, shifting further forward at the same time. In individual 

 cases (e.g., many Diptera), a considerable concentration of the 

 ventral cord is brought about by the fusion of consecutive pairs 

 of ganglia. 



The brain (supra-oesophageal ganglion) develops in the anterior 

 region of the expanded cephalic lobes. We can, at an early stage, 

 distinguish in the brain-rudiment the following sections : — 



1. Paired thickenings of the ectoderm running forward at the 



* [Burger (No. II.) finds that the ganglionic cells in Chalicodoma arise 

 similarly from neuroblasts, but the arrangement is not so regular as that 

 observed by "Wheeler in Xiphidium and Doryphora. — Ed.] 



