MUSEUM OF COMPARATIVE ZOOLOGY. 53 



sense, at least, that I understand the tenor of his criticism (p. 415) of 

 Grenacher's belief : " At present the origin of the retina of the simple eye 

 cannot be said to have been determined ; I have sought in vain for any- 

 reliable indications as to its origin. Dr. Grenacher believes it to arise 

 by a modification of the cells of the hypoderm. His arguments in favor 

 of this origin are very unsatisfactory, and apparently indicate that the 

 vitreous, and not the retinal elements, arise from this layer." 



The conclusions reached by Schimkewitsch ('84) place him also with 

 those who regard the retina as an outgrowth of the cephalic ganglia. He 

 says (p. 10) : "According to my observations, the eye of Epeira and of 

 other spiders may be divided into two quite distinct parts : one part we 

 call epithelial, the other part retinal or neural. The first embraces a lens 

 and a vitreous body, and is separated from the second by a pre-retinal 

 membrane. The retinal part is formed by a collection of terminations of 

 the fibres of the optic nerve ; each termination is formed by an enlarge- 

 ment of the fibre, which supports, in the case of Epeira, a double bacillus 

 and nuclei. The two parts [epithelial and retinal] are enveloped by a 

 membrane — a prolongation of the neurilemma of the optic nerve — which 

 merges into (se covfond) the subcutaneous connective layer and the pre- 

 retinal membrane {lame)." At p. 14 of the same paper he adds : " The 

 existence of a pre-retinal membrane is an argument — and such is also 

 the opinion of Graber — in favor of the development of the retina at the 

 expense of a neural rudiment, and not at the expense of an epithelial 

 reduplicature, as Grenacher supposes.* Besides that, we have the very 

 important observations of Bobretzky, who shows that the retina of the 

 compound eyes of the crayfish is certainly developed at the expense of the 

 neural rudiment." His general conclusion on this matter is summarized 

 in the following words : " Les couches epitheliales et mesodermiques 

 prennent aussi part a la formation des yeux, comme cela a lieu chez les 

 Vertebres." 



In his more recent paper on the embryology of spiders, Schimkewitsch 

 ('84*; does not deal with the origin of the eyes. 



The answers to the questions concerning the source of the retina and 

 the method of its formation, now furnished by Locy, seem adequate to 



* A part of the argument implied in the above quotations from Schimkewitsch 

 does not appear directly from the quotations themselves, but rests upon his interpre- 

 tation of "the membrane which merges with the pre-retinal membi'ane and with the 

 so-called subcutaneous layer." These three structures are, in his opinion, connective 

 tissue, and therefore of mesodermic origin. 



Further along in the preseut paper this view will be discussed, and an explana- 

 tion will be offered of what seems to be the cause of the author's apparent error. 



