THE MEDIAN OCELLI, STEMMATA, OR SIMPLE EVES. 513 
the outer consisting of the vitreous, and the inner of the 
retina. 
Knowing as I do that the whole substance of the hypo- 
dermic cells is frequently converted into cuticular structures, 
and having observed the manner in which the so-called vitreous 
layer becomes thinner in the Blow-fly as development advances, 
I cannot admit the importance which has been ascribed to the 
absence of a vitreous in certain forms of stemmata; and in the 
face of the direct statements of Graber that he has seen a 
vitreous in the lateral eyes of Scorpions, it appears to me it is 
simply a question of age whether the vitreous exists or is 
absent in the so-called monostichous stemmata. Neither can 
I admit the theory of Locy and Mark, that the retina is some- 
times inverted in the simple eyes of Arthropods. 
The view that the retina of the simple eye is developed from 
the hypodermis is maintained by Grenacher [216, 222]; but the 
only evidence he adduces in its favour is the arrangement of 
the cells of the vitreous in an Acilius and a Dytiscus larva. 
The arrangement represented in his figures is by no means 
convincing that his theory is correct, and can be as readily 
explained by a rapid increase of the hypodermic cells. In the 
Dytiscus eye the section is tangental, and probably has the 
same value as that of the Acilius larva, which is certainly not 
the arrangement one would expect to find if his hypothesis 
were correct, but is one which would readily result from hyper- 
trophy of the cells beneath the cornea. 
Moreover, the admission by Lankester and Bourne [229], that 
Graber’s preretinal membrane is continuous with the eye cap- 
sule, and the existence of intrusive connective tissue in the 
retina, described by the same authors, are apparently inex- 
plicable on the theory of its hypodermic origin. Nor is the 
continuity of the eye-capsule with the subhypodermic tissue 
indicative of a continuity of retinal and hypodermic cells, if 
this layer is mesoblastic, as it appears there is every probability 
it is. The so-called eye-capsule is, in fact, an endothelium, 
continuous on the one hand with the subhypodermic tissue of 
Viallanes, and on the other with the sheath of the optic nerve. 
