THE EVIDENCE OF THE ORGANS OF VISION J 3 



Further, in precisely the same way as in the case of the simple 

 retina, such a compound retina may be upright or inverted. Thus, 

 in the lateral eyes of crustaceans and insects, a compound retina of 

 this kind is formed, which is upright ; while in the vertebrates the 

 compound retina of the lateral eyes is inverted. 



The compound retina of vertebrates is usually described as com- 

 posed of a series of layers, which may be analyzed into their several 

 components as follows :— 



Layer of rods and cones \ 



External nuclear layer f retina proper j Ectodermic part 



External molecular layer -v 



Internal nuclear layer > ganglion of retina 



Internal molecular layer ) (retinal j neurodermic 



Optic nerve-cell layer \ ganglion of optic nerve ) § an g lion ' P art 



Layer of optic nerve fibres J 



The difference between the development of these two types of 

 eye — those with a simple retina and those with a compound retina — 

 has led, in the most natural manner, to the conception that the 

 retina is developed, in the higher animals, sometimes from the cells of 

 the peripheral epidermis, sometimes from the tissue of the brain — two 

 modes of development termed by Balfour 'peripheral' and 'cerebral.' 

 An historical survey of the question shows most conclusively that all 

 investigators are agreed in ascribing the origin of the simple retina 

 to the peripheral method of development, the retina being formed 

 from the hypodermal cells by a process of invagination, while the 

 cerebral type of development has been described only in the develop- 

 ment of the compound retina. The natural conclusion from this fact 

 is that, in watching the development of the compound retina, it is 

 more difficult to differentiate the layers formed from the epidermal 

 retinal cells and those formed from the epidermal optic ganglion- 

 cells, than in the case of the simple retina, where the latter cells 

 withdraw entirely from the surface. This is the conclusion to which 

 Patten has come, and, indeed, judging from the text-book of Kor- 

 schelt and Heider, it is the generally received opinion of the day 

 that, as far as the Appendiculata are concerned, the retina, in the 

 true sense — the retinal end-cells, with their cuticular rods, — is formed, 

 in all cases, from the peripheral cells of the hypodermal layer, the 

 cuticular rods being modifications of the general cuticular surface 

 of the body. The apparent cerebral development of the crustacean 



