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SYDNEY J. HICKSON. 
the Arthropods, where the nerve-end cells are comparatively 
very large and widely separated from one another. 
In the human eye, for example, the distance between the 
centres of two adjacent cones is only mm., but in Musca 
the distance between adjacent ommatidia is ^4^- mm. In 
fact the picture, as received by the nerve-end cells of the 
Vertebrate eye, is much more complete in itself than it can 
possibly be in any Arthropod eye, and consequently the latter 
possesses a much more elaborate and complete translating 
apparatus in their retina than the former possesses. 
If this view be a sound one it is necessary to give up the 
term “ optic ganglion,” as applied to terminal ganglionic 
swellings in the optic tract of animals, such as the so-called 
optic ganglia of Cephalopods and of Arthropods, and consider 
them all as forming part of the true retina. 
The observations I have made concerning the comparative 
anatomy and development of the peri-opticon may possibly 
possess an interest outside the bounds of comparative ophthal- 
mology. 
In considering the origin of the nervous system of animals 
we are able to see fairly clearly the origin of nerve-fibres and 
ganglion-cells. Thus Balfour (1) said : “ From embryology 
we learn that the ganglion-cells of the central part of the 
nervous system are originally derived from the simple un- 
differentiated epithelial cells of the surface of the body “ that 
ganglion-cells have been evolved from simple epithelial cells 
of the epidermis and that “ the primitive nerves were out- 
growths of the original ganglion-cells but the evolution of 
the central ganglia has not hitherto been investigated. 
It is, I think, evident, from both embryological and phylo- 
genetic considerations, that the formation of the central ganglia 
was subsequent to that of the ganglion-cells and nerve-fibres, 
and that they must have originated from either one or both 
of these elements. 
The characteristic feature of a ganglion is the very fine 
reticulum of nerve fibrillse which forms what I have called a 
neurospongium, with which are associated a number of larger 
