THE EVIDENCE OF THE ORGANS OF VISION 



91 





f.br.r 

 b.rn 



nl.r.g, 



» 



immm 



$?— ml 



of as composed of two ganglia. The external of these is clearly the 

 ganglion of the retina ; its cells form the nuclear layer, the striking 

 character of which, and close resemblance to the corresponding layer 

 in vertebrates, is shown by Claus' picture, which I reproduce (Fig. 40). 

 The internal ganglion with which the optic nerve is in connection 

 contains large ganglion cells, which, to- 

 gether with smaller ones, form the gang- 

 lionic layer of Berger. 



The most recent observations of the 

 structure of the compound retina of the 

 crustacean eye are those of Parker, who, 

 by the use of the methylene blue method, 

 and Golgi's method of staining, has been 

 able to follow out the structure of the 

 compound retina in the arthropod on the 

 same lines as had already been done for 

 the vertebrate. These two methods have 

 led to the conclusion that the arthropod 

 central nervous system and the verte- 

 brate central nervous system are built up 

 in the same manner — viz. by means of a 

 series of ganglia connected together, each 

 ganglion being composed of nerve-cells, 

 nerve-fibres, and a fine reticulated sub- 

 stance called by Leydig in arthropods 

 ' Punktsubstanz,' and known in verte- 

 brates and in invertebrates at the present 

 time as 'neuropil.' A further analysis 

 resolves the whole system into a combi- 

 nation of groups of neurones, the cells 

 and fibres of which form the cells and 

 fibres of the ganglia, while their dendritic 

 connections with the terminations of other neurones, together with 

 the neuroglia-cells form the 'neuropil.' As is natural to expect, 

 that part of the central nervous system which helps to form the 

 compound retina is built up in the same manner as the rest of the 

 central nervous system. 



Thus, according to Parker, the mass of nervous tissue which 

 occupies the central part of the optic stalk in Astacus is composed 



Fig. 40. — Bipolar Cells of 

 Nuclear Layer in Ketina 

 op Branchipus. (After 

 Claus.) 



f.br.r,, terminal fibre - layer 

 of retina; n.l.r.g., bipolar 

 cells of tbe ganglion of the 

 retina = inner nuclear layer ; 

 m.l., Punktsubstanz = inner 

 molecular layer ; b.m., base- 

 ment membrane formed by 

 neurilemma round central 

 nervous system. 



