54 BULLETIN OF THE 



settle these conflicting views, — so far, at least, as regards the eyes of the 

 spider-like type. While the fornaation 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 typos 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 ^os^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, I. c. 

 PI. X.), and therefore in the inverted condition of the layer are in front 

 of the retinal nuclei. | In the course of the involution the outer or thick 

 wall of the pocket becomes applied directi}'' 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 atfords 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 — diplo?,i\chon?> for an omraateum 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. 



t Compare footnote, p. 66. 



t 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 Oraber has done. 



