MUSEUM OF COMPARATIVE ZOOLOGY. 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 Grenacher (’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 is 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 znner 
cuticula 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 (‘“ preeretinale Zwischenlamelle ”) 
between the “ vitreous body ” and the retina, and, as we have seen, to lay 
stress upon its existence as an argument against Grenacher’s supposition 
that the retina was derived from the hypodermis. The question in his 
mind turned upon the direct continuity of the hypodermis (pigment) cells 
with the cell-layer forming the retina. Suchacontinuity 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. 
