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 bourrelc.t 

 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 



