MUSEUM or COMPARATIVE ZOOLOGY. 71 



I think it is sufficiently evident that this " fihrous 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- 

 laciyig fibres (compare Greuacher, '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. 



Carri^re ('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 Carriere regards his " Basalmembran " as the 

 equivalent of Graber's pre-retinal membrane, and as a cuticular structure. 



Locy's observations and the conclusions Avhich 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 inverted 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 

 (" Birmen- 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 



