THE MEDIAN OCELLI, STEMMATA, OR SIMPLE EYES. 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. 



