54 BULLETIN OF THE 
settle these conflicting views, —so far, at least, as regards the eyes of the 
spider-like type. While the formation of the retina from the epiblast, 
independently of the cephalic ganglia, determines the controversy in favor 
of those who have maintained its hypodermal origin, the method by 
which it is formed shows that none of his predecessors have in the least 
foreseen the true course of events. 
He has discovered that in both types of retina exhibited by spiders 
the retinal part of the eye is formed by an infolding. In the anterior 
median eyes of Agelena*— and probably the same is true in all spiders’ 
eyes which fall under the class called by Graber post-bacillar, — this in- 
folding gives rise to a pocket which is ultimately detached t from the 
hypodermis. The two walls of the pocket soon come into contact, so 
that this infolded, detached portion of the eye is composed of two layers. 
The layers are of unequal thickness ; and while one of them — the thinner 
and deeper — remains normal, the other, by the process of infolding, be- 
comes inverted. The cells of the thick, inverted layer are developed into 
retinal cells. The bacilli are formed at what were originally the deep 
ends of the ectoderm cells (Figs. 1, 8, 10, 20-22. Compare Locy, J. ¢. 
Pl. x.), and therefore in the inverted condition of the layer are in front 
of the retinal nuclei.t In the course of the involution the outer or thick 
wall of the pocket becomes applied directly to the deep surface of that 
portion of the ectoderm which lies immediately behind the infolding. 
This region of the ectoderm is meantime being converted into a so-called 
vitreous body. 
The inversion of the retina proper is a fact of broader significance than 
would at first appear, and it affords a satisfactory explanation of some 
of the points in the anatomy and histology of simple eyes which have 
been so earnestly discussed during the past few years. 
After Grenacher (’79) it is especially Lankester and Bourne (’83) who 
have emphasized the differences between what the latter authors have 
named monostichous and diplostichous ommatea ; but how far they still 
were from a full appreciation of the real differences is to be gathered both 
from the name employed — diplostichous for an ommateum composed of 
at least three originally distinct layers —and from the statement that 
Grenacher had shown in Myriapoda stages intermediate between mono- 
stichous and (their so-called) diplostichous conditions. From the latter 
* The conditions in the remaining eyes of Agelena are described and discussed on 
pp- 75 and 94. 
+ Compare footnote, p. 66. 
+ It seems to me more appropriate to refer the position of the bacilli to that of 
the nuclei, rather than vice versa ; and I shall therefore speak of the two types of eyes 
as pre- and post-nuclear, instead of post- and pre-bacillar as Graber has done. 
