THE EVIDENCE OF THE ORGANS OF VISION 71 
contiguous though separate, as in the lateral eyes of the scorpion. 
They may, however, come into close contact and form one single, 
large, compound eye. Such ocelli, in a very large number of cases, 
retain each its own dioptric apparatus, and therefore the external 
appearance of the compound eye represents not a single lens, but a 
large number of facets, as is seen in the eyes of insects. Owing to 
these differences, eyes have been divided into simple and compound, 
and into facetted and non-facetted. 
Yet another complication occurs in the formation of eyes, which 
is, perhaps, the most important of all: the retinal portion of the eye, 
instead of consisting of simple retinal cells, with their accompanying 
rhabdites, may include within itself a portion of the central nervous 
system. 
The rationale of such a formation is as follows: The external 
covering of the body is formed by a layer of external epithelial cells 
—the ectodermal cell-layer—and an underlying neural layer, of which 
the latter gives origin to the central nervous system. As development 
proceeds, this central nervous system sinks inwards, leaving as its 
connection with the ectoderm the sensory nerves of the skin. That 
part of the neural layer which underlies the optic plate forms the 
optic ganglion, and when the central nervous system leaves the 
surface to take up its deeper position, the strand of nerve-fibres 
known as the optic nerve, is left connecting it with the retinal cells 
as seen in Figs. 28, 29. It may, however, happen that part of the 
optic ganglion remains at the surface, in close connection with the 
retinal end-cells, when the rest of the central nervous system sinks 
inwards. The retina of such an eye is composed of the combined 
optic ganglion and retinal end-cells ; the strand of nerve-fibres which 
is left as the connection between it and the rest of the brain, which 
is also called the optic nerve, is not a true peripheral nerve, as in 
the first case, but rather a tract of fibres connecting two parts of the 
brain, of which one has remained at the periphery. Such a retina, 
in contradistinction to the first kind, may be called a compound retina. 
The optic ganglion, as seen in eyes with a simple retina, consists 
of a cortical layer of small, round nerve-cells, and an internal medulla 
of fine nerve-fibres, which form a thick network known as ‘ Punct- 
substanz,’ or in modern terminology, ‘Neuropil.’ Fibres which pass 
into this ‘neuropil’ from other parts of the brain connect them 
with the optic ganglion. 
