190 



THE COCKROACH I 



We shall now proceed to consider the development of the 

 several organs of the Cockroach. 



Nervous System. Along the middle line of the whole ventral 

 surface there is formed a somewhat deep groove-like infolding 

 of the epiblast, bounded on either side by paired solid thicken- 

 ings, which detach themselves from the epiblast (fig. 110, N} and 

 constitute the double nervous chain. In many other Insects a 

 median cord (from which are derived the transverse intergan- 

 srlionic commissures) forms along the bottom of the nervous 



o / o 



fold. This secondary median fold is very inconspicuous and 



Fig. 111. Transverse section of older Embryo of B. yermanica (abdomen). E. Epi- 

 blast ; H, kypoblast ; Ht, heart ; Gf, reproductive organs ; S, spherical granules. 



slightly developed in the Cockroach, so that the transverse 

 commissures between the developing ganglia are mainly con- 

 tributed by the cellular substance of the lateral nervous band. 

 The brain is formed out of two epiblastic thickenings which 

 occupy shallow depressions. The so-called inner neurilemma, 

 which surrounds the ventral nerve-cord, is developed as follows: 

 Along the ventral nerve- cord, and between its lateral halves, 

 a small solid cellular band (fig. 110, C/i) is developed out of the 

 mesoblastic diaphragm described above. This grows round 

 the ventral nerve-cord on all sides (fig. 112, JV'), passing also 

 inwards between the central fibrillar tract and the outer 



