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 



i- 



1 



Fig. 28. — Diagram op Formation op 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 



