480 THE XERl'OUS SYSTEM. 



consisting of two longitudinal thickenings of the epiblast, it 

 undoubtedly undergoes segmentation, and is subsequently 

 found to consist of an integumental layer of columnar cells, 

 and of deeper layers of irregularly polyhedral cells ; but these 

 deeper cells do not become directly converted into the ganglia ; 

 the latter first appear as small groups of cells imbedded in the 

 deeper layer, and only form a very small part of the primitive 

 band. Compare Kowalevski's figures [97, PI. X., Figs. 31 to 39]. 

 Possibly each ganglion originates from a single cell by rapid cell 

 division, or from a small group of cells ; but I am unable to 

 state anything definitely as to the origin of the primitive 

 ganglia. All the cells in each ganglion in my preparations are 

 precisely similar, and are very definitely separated from the 

 larger surrounding cells. It is only subsequently, when the 

 ganglia have attained considerable dimensions, that they 

 assume the form of a segmented cord on either side of a 

 very narrow median fissure. 



Quite recently Viallanes has described the manner in which 

 the ganglia are developed in the embryo of Mantis (M. rcligiosu} 

 [190]. He says: 'At first the primitive band is a simple 

 thickening of the ectoderm that is to say, a region in which 

 the cells have become columnar and augmented in volume ; 

 very soon these cells multiply and divide into two layers, a 

 superficial dermato-genetic, and a deep ganglio-genetic layer. 

 At a period which varies according to the region examined, 

 the dermato-genetic layer separates from the ganglio-genetic, 

 and becomes the hypoderm.' Viallanes further states that 

 the ganglio-genetic cells give rise to ganglion cells by division, 

 and that the fibrillar or punctate substance is developed 

 between the latter. Wheeler [191] , who follows Viallanes 

 very closely, describing the origin of the ganglia in Xiphidium 

 ensiferum, one of the American Locustidae, speaks of the 

 ganglio-genetic cells as 'large clear ncuroblasts, scattered 

 beneath the superficial cells of the blastoderm.' Such may be 

 the origin of the nervous elements ; but the appearances with 

 which I am familiar are very unlike those figured by Wheeler. 

 The appearances represented in Viallanes' figures closely 



