THE . VA'A' I 'OL'S S ) '5 TLM. 



however, a kind of neural crest is distinctly seen (Fig. 62), 

 which passes into the epiblast on either side, from which the 

 segmental nerves are developed. 



In the earlier stages, the ganglia extend the whole length of 

 the embryo, one pair in each segment, as they do in Chirononuis 

 and the Tipulida;. As development advances, the nervous 

 chain in the Blow- fly embryo becomes considerably shortened, 

 and the ganglia are drawn together and fused into the short, 

 thick conical neuroblast, which in the recently hatched larva 

 is not more than one-third the length of the body. They are 

 thus shifted from the segments in which they were originally 

 formed. At the time of the escape of the larva from the egg, 



T> - .: 



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' ~ U 'V': ! ) JBteg^ 



.;- *?/*.. W' , 



'- -.. ,v; : . .'., ..;...;" 



FlG. 62. A section of an embryo of the Blow-fly about eighteen hours old, 

 showing the neural crests and the diverticulum of the ccelom between the vential 

 ganglia. <//, the chyle stomach ; J, dorsal vessel : /, /, the intestine; in, a Mal- 

 pighian vessel ; , the neural crest ; //-, /;-, tracheal vessels. 



the segmentation of the neuroblast is very obscure, and the 

 existence of a series of ganglia corresponding to those of the 

 ventral chain, in more generalised Arthropods, is only indicated 

 by slight furrows on the surface, and by the existence of ten 

 pairs of nerves. Indeed, the number of ganglia appears to be 

 reduced to six pairs, as it is in the Pupiparae, by the atroplu 

 or great reduction fn the si/e of the ganglia, corresponding to 

 the posterior abdominal segments (compare PI. II, Figs, j, 8, 



