486 THE XERVOUS SYSTEM. 



indicate that this layer is ganglio-genetic. In the neighbour- 

 hood of the pyramidal ganglion (PI. XX I V., Fig. i, .v) and of 

 the antennal ganglion the mantle is continuous with groups of 

 large nerve cells, but whether the small round cells are developed 

 from the mantle is still, I think, doubtful ; nevertheless, I regard 

 it as probable that they are in part at least. 



Viallanes continues his description of the development of 

 the optic ganglia as follows : 



' At the same time that the layers of ganglion cells are pro- 

 duced a layer of fibrillar substance appears, at first on the 

 inner surface of the ganglion layers; it is, so to speak, secreted 

 (excuse the comparison) by the free surface of the ganglion 

 layers. The fibrillar substance once produced is rapidly in- 

 creased, and at the same time advances into the midst of the 

 ganglion cells by a kind of invagination. The penetration of 

 the fibrillar substance into the layer of ganglion cells appears 

 to me to be the result of their multiplication, by which the in- 

 creasing mass of cells tends to enclose the fibrillar substance.' 



That the central white substance originates from cells is 

 indubitable, but it appears in the Blow-fly long before any 

 nerve cells have been produced from the mantle layer. In the 

 newly-hatched larva the fibrillar substance of the brain and 

 ventral cord is already developed (Fig. 64), and both are sur- 

 rounded by a multiple layer of ganglio-genetic cells. In PI. II., 

 Fig. 7, I have already represented this stage, and there is no 

 trace of a mantle layer. Since publishing this figure I have 

 obtained sections showing the mantle layer enclosing a large 

 cavity in front of the brain, and not yet applied to the hemi- 

 sphere, but in immediate relation with the dermal head-discs. 



Viallanes' own figures seem to me to indicate that there are 

 numerous cellular elements in the region from which the brain 

 is developed long before the mantle layer has been laid do^vn 

 over their surface, and in one the white substance is apparently 

 represented amongst these cells [190, Fig. 8, near/) o] . 



It appears to me that there is evidence, although not 

 conclusive evidence, in favour of a totally new view of the 

 manner in which the preoral ganglia are developed a view 



