THE LENS-PROBLEM. 22Q 



been removed, the latter was able to stimulate the formation of 

 lenses, because there being no gap between it and the epidermis 

 transplanted over it, there was no chance for an obstruction of 

 its contact with the latter by ingrowing mesenchyme. It is, 

 besides, needless to say that it is difficult to understand just why 

 Spemann in the experiments in which the epidermis was trans- 

 planted over a defective eye vesicle assumes a sharp predeter- 

 mination to form lenses and not so, if transplanted over an unin- 

 jured eye vesicle. 



Summarizing briefly we find that critical sifting of the results of 

 Spemann 's experiments on Rana esculenta discloses no warrantable 

 evidence whatsoever for the presence in this frog species of an early 

 predetermined lens-forming part of the epidermis capable of dif- 

 ferentiation into a lens without the stimulus from an optic vesicle 

 or at least a fragment of it. 



Practically all of these experiments were performed by Spe- 

 mann also on Bombinator pachypus. The results obtained in this 

 species differed according to Spemann ('07, '12 ff) from those 

 obtained in the pricking experiments in Ranafusca and also from 

 the above-noted results in Rana esculenta. They indicate, 

 Spemann believes, that Bombinator in regard to the ability to 

 form a lens by self-differentiation occupies a position intermediate 

 between Rana fusca and Rana esculenta. 



It is evident, I believe, that were this really so, i. e., if the 

 supra-ocular epidermis of Bombinator possessed a certain degree 

 of this ability and only needed the stimulus from the optic vesicle 

 as a complementary aid for the differentiation of a lens, a serious 

 stumbling block would here confront every attempt at the solu- 

 tion of the lens-problem. 



Fortunately, however, Spemann 's observations seem to call 

 for an entirely different interpretation of the results. 



Thus in the excision experiment (right foveola optica) the 

 latter prove very decidedly, I believe, that no lens can be formed 

 in the absence of an optic anlage or on its failure to come tinto 

 contact with the overlying epidermis. For, out of the forty-six 

 embryos operated upon in twenty of them in which an optic 

 fragment remained (owing to incomplete excision), a lens de- . 

 veloped on the side of operation. Of the remaining twenty-six 



