GROWTH OF THE BRAIN AND EYES 



567 



cord," and give rise to the transverse commissures connecting the 

 ganglia. 



Wheeler has detected in the rudiment of the ventral cord of several Orthoptera, 

 on the upper surface of the lateral cords, four large cells which he calls neuro- 

 blasts (Figure 541, n\ ?i 4 ), from 

 which cells arise by budding and 

 become arranged in vertically ar- 

 ranged layers or pillars (z). Graber 

 has observed them in Stenobothrus 

 and Viallanes in Mantis. These 

 neuroblasts are only present in the 

 inter-ganglionic region, and soon 

 move back to the hinder side of the 

 transverse commissures. 



n 



m 



FIG. 541. Transverse section through the rudi- 

 ment of the ventral nervous cord of Xiphidium : /, 

 fibrous mass; m, neuroblast-cells of the median 

 cord ; i-ft 4 , neuroblasts of the lateral cord ; z, 

 pillar of ganglion-cells arising from the neuroblasts. 

 After Wheeler. 



At first there is a pair of 

 ganglia to each of the 16 trunk- 

 segments of the embryo, but 

 afterwards these become more 

 or less fused together; thus 

 those of the three gnathal segments unite to form the subossophageal 

 ganglion of the adult, and the last abdominal ganglia are fused 

 together and move a little anteriorly (see also pp. 227, 228). 



Development of the brain. The supra-oesophageal ganglion is due 

 to the spreading out of the procephalic lobes. The rudiment of the 

 brain is due to a thickening of the ectoderm on the sides of the 

 mouth and of the fore-head, this expansion of germinal brain-cells 

 being the direct continuation of the primitive rolls or strips, and 

 which finally becomes differentiated into the protocerebrum, deuto- 

 cerebrum, and tritocerebrum, as stated on p. 228. 



The ganglion opticum, now regarded as a part of the compound 

 eye, arises as an ectodermal thickening on each side of the rudi- 

 mentary brain. The optic ganglion belongs exclusively to the fore- 

 most division of the brain (see also p. 227). 



Development of the eyes. Compound eyes do not appear until the 

 beginning of pupal life, the single Tye (ocellus) being the primi- 

 tive organ of vision. The ocellus of Acilius, according to Patten, 

 arises as a pit or depression of the ectoderm (Fig. 542). The 

 long hypodermal cells which form the walls of this pit or hollow 

 are arranged in a single layer, and bear at their free ends a striated 

 cuticular edge (c), while from their inner or basal end arise the fibres 

 destined to form the common optic nerve. 



At a later stage (Fig. 542, _B), the eye-pit is closed over, the 

 edges growing over and covering the deeper part of the eye. In this 



