150 PROCEEDINGS OF THE ACADEMY OF [Feb., 



The innervation of the ommatidium is a question over which there 

 has been much discussion, and various views have been put forth. Tlie 

 views can, however, be classed into two groups: those which make the 

 cone cells the nerve-ending, and those which find the terminations in 

 the retinula. It has been shown conclusively by numerous investiga- 

 tors that the cone has nothing whatever to do with receiving light 

 stimuli, and it would be useless to take up the arguments against this 

 view, any more than has been done in showing that in the development 

 the cone and rhabdome are separate. 



Those who hold that the retinula is the nerve-ending of the omma- 

 tidium have not always been able to show in a satisfactory manner 

 just how this innervation takes place. On this point two views have 

 been held: (1) that the retinula is innervated by nerve fibrils from 

 the retinular ganglion which run into the retinular cells or rhabdome, or 

 (2) that the retinular cells are themselves ganglionic epidermal cells 

 which send in nerve fibres to the retinular ganglion. From the de- 

 scription which has preceded it is evident that the second of these 

 \dews is the one here held for the eye of the bee. Before going into a 

 detailed descripton of the nervous elements in the cells concerned, 

 let us first examine the problem. 



In the first place, it seems reasonable to assume that during the 

 coiu-se of the evolution of light-perceiving organs the first condition 

 was that in which certain cells of the hypodermis became sensitive to 

 light, or possibly heat, through the accumulation of pigment or some 

 other change in the cytoplasm. Such cells would arise before there 

 were any cells in the central nervous system to receive their nerve 

 stimuli, and it may be assumed without danger that such cells would 

 send in processes to the centrally placed nerve cells, when the time for 

 nerve connections arrived, rather than that the nerves arose from the 

 central nervous system. In other words, the peripheral nervous 

 system is older than the central nervous system which elaborates the 

 impulses, and on hypothetical grounds, a basis which is rather unsafe 

 in zoology unless backed up by observations, we may assume that the 

 innervation is centrad. 



From the standpoint of embryology, we find that the eye epidermis 

 is formed and even the ommatidia are differentiated before the retinal 

 ganglion cells have assumed their adult position or are connected with 

 the optic ganglia. Not only that, but the strands of cytoplasm which 

 become the nerves of the ommatidia arise from the retinula cells 

 and grow centrad. 



In the adult condition we find that the nerve filjrcs are continuous 



