MUSEUM OF COMPARATIVE ZOOLOG\. 67 



I have referred especially to the ocelli of Hymenoptera because of the 

 evidence of a third layer, and the certainty of there being a " vitreous " 

 which undergoes a great reduction during the development of the eye. 

 Even if a " vitreous " should in some instances appear to be wanting in 

 the adult, the condition could be fairly explained as a result of ultimate 

 atrophy. The evidence for the existence of a third layer is in most cases 

 still wanting. "When Grenadier (79, p. 57) claimed a substantial agree- 

 ment in the morphology of the ocelli of insects and the eyes of spiders, 

 he based his conclusion on the presence of two distinct cell-layers, — a 

 vitreous and retina. With the present knowledge of the development in 

 the case of spiders, it again becomes an open question whether the mor- 

 phological change in insects follows the same fundamental plan. It "S not 

 impossible that there are among insects two methods of development for 

 the ocelli, — one with, the other without, retinal inversion. A conspicu- 

 ously reduced " vitreous," and the probable existence of a distinct post- 

 retinal layer of cells in Hymenoptera, inclines me to the opinion that in 

 some cases, at least, there is an inversion. 



One of the questions which is most intimately connected with that of 

 the origin of the retina concerns the nature and significance of the pre- 

 retinal membrane. In connection with this I shall consider the inner 

 cuticala or basement-membrane of the hypodermis and the " sclera." 



Graber ('79, pp.64-67) was the first to call attention to the existence of 

 a homogeneous cuticula-like membrane (" prseretinale Zwischenlamelle ") 

 between the " vitreous body " and the retina, and, as we have seen, to lay 

 stress upon its existence as an argument against Grenadier's supposition 

 that the retina was derived from the hypodermis. The question in his 

 mind turned upon the direct cordinuity of the hypodermis (pigment) cells 

 with the cell-layer forming the retina. Such a continuity being precluded 

 by the presence of his pre-retinal membrane, the inference of a hypoder- 

 mal origin for the retina became for him untenable. 



Grenacher subsequently ('80, pp. 429, 430) conceded the existence of 

 such a structure in the case of scorpions and spiders, but was unwilling 

 to follow Graber in his generalization that all " Stemmata " possess this 

 membrane. Unable to disprove Graber's claims in the case of Dytiscus by 

 a re-examination of the subject, he was still unwilling to give them any 

 weight, because Graber " claimed with equal certainty the existence of 

 such a cuticular membrane for Myriapods, but assigned to it an entirely 

 impossible location." But the problem of reconciling a pre-retinal mem- 

 brane with the supposed hypodermal origin of the retina, was not attempted 

 by Grenacher. 



