THE DEVELOPMENT OF THE NERVOUS SYSTEM. 485 



' The ganglio-genetic cells, which at first are few in number, 

 increase and multiply, without, however, changing their char- 

 acter, and always remaining disposed in a single layer. 



' In the seventh stage these cells cease to produce elements 

 similar to themselves, and give rise on their deep surface to 

 much smaller elements very poor in protoplasm, with nuclei 

 easily stained by carmine which may be designated as ganglion 

 cells; as soon as these are formed they themselves multiply 

 very actively. Owing to this multiplication, the first proto- 

 cerebral lobe becomes a mass of cells convex externally and 

 concave within. 



' It consists, then, of two layers of cells which are very 

 distinct, a superficial layer on its convex surface of ganglio- 



4SKSV;<v\&rasi 



Sg^SMiP' 



L* J . ^-: i. i*s %&>* rs 



FlG. 63. A sagittal section through the outer part of the hemisphere of the adult 

 larva showing the invagination of the mantle layer from which the retina of the 

 imago is developed, d, invaginated layer surrounding the retinal rudiment ; 

 e', epithelial mantle layer ; /, part of the pyramidal ganglion ; /, superficial cells 

 of the optic ganglion ; o, central stroma of optic ganglion ; p, group of small 

 cells invaginated in the cavity of the rudimentary retina. 



genetic cells and a deeper portion consisting of numerous 

 layers of ganglion cells.' 



So far Viallanes' description appears to be extremely clear, 

 and to correspond closely with the phenomena observed in the 

 Blow-fly by myself. It is possible that the deeper layers of 

 cells are formed from the mantle layer. 



In two regions I have seen appearances which seem to 



