THE EVIDENCE OF THE ORGANS OF VISION 93 



The comparison of the arthropod compound retina with that of 

 the vertebrate shows, as one would expect upon the theory of the 

 origin of vertebrates put forward in this book, that the latter retina 

 is built up of two ganglia, as in the more primitive less specialized 

 crustacean forms. The modern description of the vertebrate retina, 

 based upon the Golgi method of staining, is exactly Parker's descrip- 

 tion of the simpler form of crustacean retina in which the ' neuropil ' 

 of the first ganglion is represented by the external molecular 

 layer, and that of the second ganglion by the internal molecular 

 layer ; the three sets of neurones being, according to Parker's 

 terminology : — 



1. The neurones of the first order — viz. the visual cells — the 

 nuclei of which form the external nuclear layer, and their long 

 attenuated processes form synapses in the external molecular layer 

 with 



2. The neurones of the second order, the cells of which form the 

 internal nuclear layer, and their processes form synapses in the 

 internal molecular layer with 



3. The neurones of the third order, the cells of which form the 

 ganglionic layer and their neuraxons constitute the fibres of the optic 

 nerve which end in the optic lobes of the brain. 



Strictly speaking, of course, the visual cells with their elongated 

 processes have no right to be called neurones : I only use Parker's 

 phraseology in order to show how closely the two retinas agree even 

 to the formation of synapses between the fine drawn-out processes of 

 the visual cells and the neurones of the ganglion of the retina. 



The Eetina of the Lateral Eye of Ammoccetes. 



As in the case of all other organs, it follows that if we are dealing 

 here with a true genetic relationship, then the lower we go in the 

 vertebrate kingdom the more nearly ought the structure of the retina 

 to approach the arthropod type. It is therefore a matter of intense 

 interest to determine the nature of the retina in Ammoccctes in order 

 to see whether it differs from that of the higher vertebrates, and if 

 so, whether such differences are explicable by reference to the structure 

 of the arthropod eye. 



Before describing the structure of this retina it is necessary to 



CD 



clear away a remarkable misconception, shared among others by 



