MUSEUM OF COMPARATIVE ZOOLOGY. 71 
I think it is sufficiently evident that this “fibrous membrane” in Salticus 
cannot be considered the morphological equivalent of the pre-retinal mem- | 
brane originally described by Graber ; for if it were, it would be the only 
known case in which the pre-retinal membrane was composed of inter- 
lacing fibres (compare Grenacher, ’78, Figs. 25, 27), to say nothing of the 
occasional presence of nuclei within it. Hence, while I agree with the 
implied conclusion of Lowne that the vitreous layer and the retina are 
separated by a cuticular structure, I regard his reasons as altogether 
uncritical, and such as would lead, if logically pursued, to an entirely 
different conclusion. 
Carriére (’85, p. 187), considering it probable that the two layers of the 
monostichous eyes have originated by a process of delamination, as in the 
compound eyes, finds it in no way remarkable —even though the sepa- 
ration is much more distinct than in the latter case—that the outer 
layer of the monostichous eye (a genuine epithelium) develops a “ Basal- 
membran” after the manner of the ordinary epithelium of Arthropods. 
“But this membrane not only separates, it also joins the upper with the 
lower layer ; at least I have never met a case in which the two layers 
had become separated from each other.” Although not precisely stated, 
there can be no doubt that Carriére regards his ‘“‘ Basalmembran” as the 
equivalent of Graber’s pre-retinal membrane, and as a cuticular structure. 
Locy’s observations and the conclusions which directly result from 
them not only place the retina in a more satisfactory relation to the hypo- 
dermis, but also afford at the same time a fair explanation of the condi- 
tion and mutual relations of sclera, pre-retinal membrane, and the internal 
_cuticula of the hypodermis. It now becomes probable — unless in spe- 
cial cases the reverse is proved by direct observation —in all those 
instances where a pre-retinal membrane is demonstrable in the adult 
“‘stemma,” first, that the retina has been produced by an involution of 
the ectoderm (hypodermis), which has znverted the more superficial of the 
two infolded cell-layers ; and consequently, secondly, that the eye is not 
simply two-layered, as supposed by Grenacher as well as all subsequent 
observers, but is really three-layered (triplostichous). 
In the light of this process of involution the deep cuticular layer 
(“‘ Binnen-Cuticula,” Graber) appears in readily appreciable relations. 
Whether as an “inner cuticula” to the permanent hypodermis and the 
pigment-cells, as the so-called sclera which invests the retinal bulb, or as 
a pre-retinal membrane, it is really one and the same thing. These three 
structures have a like origin, —they are the continuous product of the 
basal ends of ectoderm cells ; and the pre-retinal membrane alone requires 
