4yo THE NERVOUS SYSTEM. 
continuity of the structure in question with the mantle layer 
renders them in the highest degree improbable. In my pre- 
parations the continuity of this layer with the ganglion cells of 
the pyramidal ganglion on the one hand, and with those of the 
antennal ganglia on the other, is indubitable, and its continuity 
with my retinal disc is no less certain. 
Of the latter Viallanes says, ‘ The bourrelet perilaminaire 
should be considered as the remains of the layer of ganglio- 
genetic cells.’ The evidence on which he regards one of these 
structures as different from the other is not very apparent, but 
I suppose he takes such opposite views of their nature because 
he thinks that the bourrelet intraganglionnaire is derived from 
rhe 
: rane es > 
5 YU aa ag 
. A) \ Reeser es 
FRY)». NRE SNE 
= gy 
% C 
C 
Fic. 66.—A diagram similar to Fig, 65, ata later period. ¢,, space between the brain 
and head capsule ; ¢, cavity of the hemisphere ; 4, hemisphere ; /, leg disc ; 7, 
neural crest of hemisphere ; 7, neural crest of ventral ganglion; 0, optic disc ; 
rv, roof of the cerebral vesicle ; 77, probable origin of the retinal epithelium or 
retinal disc ; s, stomodeeum ; vc, ganglion of the ventral chain. 
an invagination of the dermato-genetic layer. In this I think he 
has fallen into an error, which is due to his regarding the whole 
of the external layer of cells as dermato-genetic, and he has 
probably mistaken the involution of a purely nervous layer for 
an open integumental invagination. 
Sections of the Neuroblast of the larva show that its structure 
