THE EVIDENCE OF THE ORGANS OF VISION 73 
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 \ retina proper } Hetodermic part 
External molecular layer 
Internal nuclear layer ganglion of retina : ; 
Internal molecular layer retinal \ neurodermic 
Optic nerve-cell layer ganglion of optic nerve ganglion part 
Layer of optic nerve fibres 
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 
