THE DEVELOPMENT OF THE NERVOUS SYSTEM. 489 
I have already given a diagrammatic figure which will serve 
to represent the above views, and the diagrams (Figs. 65 
and 66) will serve still further to elucidate them. It must be 
remembered, however, that these are merely hypothetical 
diagrams, and are not figures of actual sections. 
It is worthy of note that the mantle, or roof of the vesicle, 
is in intimate relation with the pyramidal ganglion, the nerves 
to the ocelli, the optic ganglion, and the antennal ganglion, 
so that it probably produces all those parts of the brain which 
cannot be regarded as central ganglia. 
Two portions of the mantle layer have attracted the atten- 
tion of previous observers—the deep-seated involution in the 
posterior internal tract of the hemisphere, from which the 
numerous cells of the pyramidal ganglion and the small cells 
on the posterior surface of the optic ganglion apparently origi- 
nate, the bourrelet intraganglionnaire of Viallanes; and the 
part which I have termed the retinal disc, which is the 
bourrelet perilaminaire of Viallanes, the invagination of which 
is figured by both Viallanes and Wheeler. When I stated 
(p. 326) that it had not been previously observed, I had not 
seen their recent memoirs. 
Viallanes says of the former: ‘We have described under 
this term (bourrelet ectodermique intraganglionnaire) a tran- 
sitory structure, which has the most intimate relations with 
the nervous system, whilst it takes no part in the constitution 
of the latter. 
‘In the course of development, at a point in the procephalic 
lobe near the optic disc, the ectoderm is invaginated like the 
finger of a glove; this invagination, which is the bourrelet 
intraganglionnaire, insinuates itself between the internal me- 
dullary mass and the external medullary mass. Then this 
invagination becomes strangulated at its point of origin, and 
separates from the ectoderm; later it enters into degeneration 
and disappears. 
‘ This formation perhaps represents a trachea, or a transitory 
cephalic gland.’ 
I take exception to all these conclusions; the identity and 
