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 (neuroglia). 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 f ovea 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 



