360 E. I. WERBER 



tion. Thus, for instance, he has observed that the consistency 

 of the embryonic tissues varies with the different frog species, 

 the substance of embryos of some species being more rigid and 

 thus easier to operate on, while in other species it may be very 

 sticky and offer great difficulty to mechanical separation of ec- 

 todermal epithelium from underlying cell layers. I am there- 

 fore inclined to conclude that owing to this difficulty in many 

 experiments in which Spemann attempted the complete ehmina- 

 tion of an eye vesicle, some fragments of the latter were left 

 attached to the epidermis, from which they eventually stimu- 

 lated the formation of lenses. Such fragments may be too mi- 

 nute for detection with the aid of the binocular microscope and 

 too small to differentiate histologically; yet they may be capable 

 of furnishing the stimulus for the lentogenic reaction, for which 

 perhaps only a very few potential retinal cells may suffice. This 

 might account for the development of the independent lenses in 

 such experiments of Spemann where an optic vesicle has been 

 removed, while in the species in which no such lenses have de- 

 veloped, although the same operation had been performed, it is 

 evidently easier to separate with perfect accuracy the optic vesi- 

 cle from the overlying epidermis and to remove it in such a man- 

 ner that no traces of it are left. In view of Spemann's well 

 known skill in such minute operations and his scrupulous care, 

 we may, I think, expect that he will sooner or later be able" to 

 confirm this interpretation of the divergence in the results of 

 these as well as other experiments (transplantation of epidermis 

 from a posterior region of the embryo onto an eye vesicle from 

 which the epidermis has been removed — Spemann, '12). 



If it were (and perhaps it some day will be) possible to control 

 the difficulties spoken of, it would most likely be found that 

 there are no exceptions to the rule that the lens of the verte- 

 brate eye cannot originate independently of a stimulus from op- 

 tic cup substance. The difficulty presented to the lens-problem 

 by Mend's and Stockard's observations may now be considered 

 as no longer existing. For the 'independent' lenses recorded by 

 these authors in teratophthalmic embryos are due solely to 'in- 

 fection' of the ectoderm with particles of blastolyzed optic cup 



