THE EVIDENCE OF THE ORGANS OF VISION 69 
hypodermal cells of A elongate to form the layer known by the name 
of corneagen, or vitreogen, the cells of B remaining small and forming 
the pre-retinal layer of cells. The large optic nerve end-cells of the 
retinal layer, C, take up the position shown in the figure, and their 
cuticular surface becomes modified to form rods of varying shape 
called rhabdites, which are attached to the retinal cells. Frequently 
the rhabdites of neighbouring cells form definite groups, each group 
being called a rhabdome. Whatever shape they take it is invariably 
found that these little rods (bacilli), or rhabdites, are modifications of 
the cuticular surface of the cells which form the retinal layer. Also, 
as must necessarily be the case from the method of formation, the 
optic nerve arises from the nuclear end of the retinal cells, never from 
Fic. 28.—D1aGRam OF FORMATION OF AN UPRIGHT SIMPLE RETINA. 
the bacillary end, As in the case first mentioned, so in this case, the 
light strikes direct upon the bacillary end of the retinal cells; such 
eyes, therefore, are eyes with an upright retina. 
It may happen that the part invaginated is the optic sense-plate 
itself, as would be the case if in the former figure, instead of C, the 
part B was modified to form a sense-plate. This will give rise to 
an eye of a character different from the former (Fig. 29). The optic 
nerve-fibres now lie between the source of light and the retinal end- 
cells, the layer A as before forms the cuticular lens, and its hypo- 
dermal cells elongate to form the corneagen ; there is no pre-retinal 
layer, but, on the contrary, a post-retinal layer, C, called the tapetum, 
and, as is seen, the light passes through the retinal layer to the 
