62 BULLETIN OF THE 



reconciling this condition of affairs with the tj'^pical one-layer condition 

 of the ommateum ; he seems to consider it, however, as only a phase in 

 the process of formation, which is insufficient to decide whether the eye 

 is to be regarded as a one-layer or a two-layer structure ; for he says : 

 " Which of these two conditions, which are alternately realized in the 

 various phases of the life of the individual, shall we assume as the pri- 

 mary, in order to refer to it the other condition (a thing which presents 

 in itself no difficulty) 1 Here, I believe, the observation of the first rudi- 

 ment of the development can alone give a reliable answer; I at least feel 

 incapable of deciding solely upon the hitherto accumulated facts." 



It might have been unwise for Grenadier, and it may be even now 

 rash for one to hazard a conjecture as to which was the primary condi- 

 tion ; but in view of what is now known about spiders' eyes, I think the 

 evidence favors the conclusion that the exceptional cases present the 

 more primitive condition. One or the other of two things is likely to 

 have taken place, — either the retina was formed by an involution which 

 allowed the " vitreous " to be from the first a continuous laell-area, or the 

 retina resulted from a depression of the hypodermis, followed by a ring- 

 like ingrowth of vitreous cells from the margins of tlie depression. The 

 obliquity of the axes of the vitreous cells, as seen in the finished eye, 

 might suggest the probability of simple ingrowth ; but in these excep- 

 tional growing eyes, the continuity of the layer, its nearly uniform thick- 

 ness, and the very slight oliquity of the central cells, while not absolutely 

 incompatible with such an origin, appear to me more favorable to the 

 supposition of a primitively uninterrupted vitreous layer. There is still 

 a wide difference between the one-layer condition figured by Grenadier 

 for Dytiscus larvae, and the completed eye of Scolopendra. If, as seems 

 probable, Grenacher is right in supposing the exceptional individuals of 

 Branchiostoma to have been engaged at the time of capture in the con- 

 struction of lenses, the lateral displacement of the vitreous cells had 

 probably only just begun; but even when completed, the "vitreous" 

 and retina still continue to form two essentially distinct cell-layers. 



Graber has claimed the existence of a pre-retinal membrane in Myria- 

 pods; but Grenacher asserts that he assigned to it an impossible position. 

 It is true Grabor has not carefully described, nor very precisely repre- 

 sented it ; but I fail to understand how it was possible fur Grenadier to 

 speak of it as located in an impossible place. However inaccurately 

 Graber may have described the cell-layers which constitute " vitreous " 

 and retina, they certainly are in contact, even according to Grenadier's 

 own description ; and it is along this region of contact that I understand 

 Graber to have located the pre-retinal membrane. Even Grenacher's 



