324 INSECTA. 



invagination (rudiment of the oesophageal commissures) on to the 

 cephalic segment, and from the first are directly connected with 

 the rudiment of the brain which develops from a thickening of the 

 cephalic lobes. This character has recently been specially emphasised 

 by Patten (No. 67), and has also been maintained by Heider 

 (No. 38) and Graber (No. 30) against Will (No. 97), who considers 

 that the brain-rudiment of the Aplridae (neural plate) arises inde- 

 pendently and becomes connected with the rudiment of the ventral 

 chain of ganglia through the oesophageal commissures which arise 

 only secondarily as in the Crustacea (cf. Vol. ii., p. 160, footnote). 



The widenings of the primitive swellings in the segments give 

 rise to the ganglia of the ventral chain, and the inter-segmental 

 constrictions to the paired longitudinal commissures. 



Transverse sections (Fig. 158 C, p. 321, and Fig. 135, p. 273) 

 show that the ectoderm becomes multilaminar in the region of the 

 primitive swellings (pw). At a later stage, the lower layers separate 

 by delamination from the superficial layers (Fig. 158 D-F, s) and 

 form the so-called lateral cords, i.e., the rudiments of the longitudinal 

 strands of the ventral chain of ganglia. The primitive groove (pr) 

 deepens meantime and forms an invagination extending between the 

 lateral cords. The cells at the base of this invagination represent 

 the so-called middle cord and give rise in the middle of the segments 

 to the transverse nerve-commissures of the different pairs of ganglia 

 (Hatschek). 



With regard to the condition of the middle cord in the inter-ganglionic 

 region, opinions are still divided. Hatschek's view that the primitive groove 

 flattens out in this region, its wall being used up entirely for the formation 

 of hypodermis, has been generally accepted, but Graber maintains (No. 30) 

 that in this region also a median cord splits off which degenerates at a later 

 stage. 



The nerve -fibrillae arise first on the inner or basal surface of the lateral 

 strands and the middle strand. Secondarily, by shifting their position, they 

 become enveloped by ganglionic cells (cf. on a similar condition in the Crustacea, 

 Vol. ii., pp. 160 and 161). 



Leydig stated that, in many Insects, there is a double transverse commissure 

 in each of the ganglia of the ventral cord, and a corresponding double rudiment 

 has been repeatedly shown to exist in the embryo (Patten, No. 66 ; Ayers, 

 No. 1 ; Heider, No. 38 ; Wheeler, No. 95 ; Graber, No. 30). No detailed 

 accounts have as yet been given of the manner in which the nerves given off 

 peripherally by the ganglia of the ventral cord arise. 



The ventral diaphragm bridging over the ventral ganglionic chain (Fig. 

 167 A, dv, p. 340), which has been observed in many Insects, is derived by 

 Korotneff (No. 47) from the mesoderm, but Heider (No. 38) believes that 

 he was able, in Hydropltilus, to trace its origin back to ectoderm-cells lying 

 laterally to the rudiments of the ganglia. 



