492 THE NERVOUS SYSTEM. 
remain spheroidal, but the lateral ganglion, which becomes 
the optic ganglion, is by far the larger,’ and he adds: ‘On 
the fifth day of the pupa the optic ganglion is an almost 
spherical organ, nearly twice the size of the inner or supra- 
cesophageal ganglion.’ 
This description is perfectly accurate as far as surface-views 
are concerned, but Weismann was unaware of the previous 
existence of the optic ganglion imbedded within the hemi- 
sphere. The previous condition of this organ was first de- 
scribed and figured by Viallanes (27, Pl. XVI. Fig. 8], so that 
it is evident that the optic ganglion is not formed as Weis- 
mann supposed, by the segmentation of a cellular mass 
into two ganglia, but that it is developed from a pre-existing 
structure, which is evaginated from the interior of the hemi- 
sphere. The first stage in the evolution of the optic ganglia 
is characterised by the rapid growth of the retinal disc in the 
interior of the hemisphere, beneath the optic stalk of the 
eye-disc; and this structure soon becomes a cup-like pro- 
jection. As development progresses, and the optic ganglion 
enlarges, the cup becomes convex instead of concave on its 
outer surface, and spreads over the outer surface of the now 
rapidly-increasing spheroidal ganglion. 
During the evolution of the optic ganglia the crura and the 
central fibrillated stroma rapidly increase in size, and the 
trabeculz are seen penetrating the group of cells which be- 
come the corpora fungiformia; whilst the large group of cells, 
from which the olfactory lohes are developed, have become 
very conspicuous on the inner and antero-inferior surface of the 
hemisphere. As has been already stated, all these structures 
are present in the larva, so that the view that the whole neuro- 
blast, as it exists in the larva, represents the functionally active 
nervous system at that stage, and that it undergoes a complete 
change, falls to the ground; and, as has been already stated, I 
think the neuroblast must be regarded as an imaginal rudi- 
ment, just as the imaginal discs are, from which the several 
parts of the brain of the imago are developed. 
The Peripheral Nerves.—This view is still further supported 
