482 THE NERVOUS SYSTEM. 
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 Chironomus 
and the Tipulidae. 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 ftom the egg, 
Fic. 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 ventral 
ganglia. c/, the chyle stomach ; d, dorsal vessel; 2, ¢, the intestine ; 7, a Mal- 
pighian vessel ; 7, the neural crest; ¢”, 47’, 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 Pupipare, by the atrophy 
or great reduction in the size of the ganglia, corresponding to 
the posterior abdominal segments (compare Pl. II, Figs. 7, 8, 
