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 nucleat 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 
8. 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 RETINA OF THE LATERAL EVE OF AMMOCGTES. 
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 onght 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 Ammoccetes 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 
clear away a remarkable misconception, shared among others by 
