492 I HI-'. XI'.Rl'OUS SYSTI-.M. 



n main spheroidal, but the lateral ganglion, which becomes 

 the optic -an-lion, 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 Yiallanes [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 

 trabecuhe are seen penetrating the group of cells which be- 

 come the corpora fungiformia ; whilst the large group of cells, 

 from which the olfactory lobes 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 



