200 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
called outer granular (nuclear) layer, separated by an outer 
‘molecular’ (reticular) layer of interlacing dendrites from the 
inner granular layer. This is ganglionic in character and is con- 
nected by the inner molecular layer with the rest of the ganglionic layer 
which lines the inside of the retinal cup. 
In order to understand the latter layer and the relations of the optic 
nerve, an account of the development is necessary. At first the retinal 
layer is comparatively thin, but it increases in thickness, in part by a 
multiplication of cells, in part by their increase in length and the devel- 
opment of the dendrites of the molecular layers. Each cell of the inner 
layer (the one turned toward the lens) also develops an axon which 
runs over the free surface of the cells to the chorioid fissure, passes 
through this and along the ventral groove of the optic stalk to the 
diencephalon. 
As will readily be understood, it is these fibres and not the optic 
stalk which form the optic nerve (p. 169). When the chorioid fissure 
closes, the nerve appears to leave through the centre of the retina, 
and as this part contains no sense cells, the point of exit constitutes 
the ‘blind spot’ of physiological works. Besides the cells already 
mentioned the retina contains supporting or radial cells, like other 
sense organs or like the brain itself (meuroglia). These extend through 
from the nerve fibres to the basal membrane. Either rods or cones 
may be absent in isolated groups of vertebrates. Usually there is a 
spot, the macula lutea (yellow spot) or fovea centralis at the centre 
of the retina where vision is most distinct. Here the rod and cone cells 
are shorter and more crowded than elsewhere. 
Here may be mentioned a point of morphological importance. It will be 
recalled (p. 138) that the ependymal surface of the brain corresponds to the external 
surface of the ectoderm of the rest of the body. Therefore, as a glance at fig. 204 
will show, the rods and cones are on the primitively outer and the ganglion cells and 
nerve fibres are on the deeper surface of the ectoderm. Hence rods and cones 
correspond to the percipient cuticular structures of other sensory organs like the 
lateral line, taste buds and the like. Before it can affect the sensory cells the light 
has to traverse the whole of the retina and then the nervous impulses have to 
pass back through the same layers to reach the optic nerve. This constitutes an 
‘inverted eye’ and, with the exception of a few molluscs, it is unknown, except in 
the vertebrates: A comparison with the parietal eye of reptiles (fig. 151) is very 
instructive. 
The cavity between lens and retina is filled with a semisolid vitre- 
ous body, the origin of which is in dispute. In mammals blood-vessels 
