22O E. I. WERBER. 



upon the contact of the optic vesicle with the supra-ocular 

 epidermis from which it arises. It will be recalled also that this 

 view was contested by Mencl ('03, '08), King ('05) and Stockard 

 ('io) and that Spemann himself has in later papers ('08, '12) 

 modified his opinion and abandoned his generalizations. He now 

 thinks that more extended experiments in which various methods 

 were employed on several frog species prove that in some species 

 the lens cannot develop if the optic vesicle fails to come into con- 

 tact with the overlying epidermis while in one species the epi- 

 dermis is capable of giving rise to lentiform bodies and even well- 

 differentiated, unmistakable lenses without such contact. 



Such conditions would, indeed, be so contrary to expectation 

 that it well behooves us to inquire into the validity of Spemann's 

 self-contesting evidence as well, as of the evidence brought forth 

 by Mencl, King and Stockard for the independent development 

 of the lens. 



I have already (Werber 'i6c) pointed out some possible sources 

 of error which would invalidate the conclusions of these authors 

 and on re-reading the entire series of Spemann's interesting 

 papers on the subject I have been struck by the complete absence 

 of any real counter-evidence against the conclusions of his earlier 

 ('oi, '03) excellent work. 



Spemann's first experiments were performed on Rana fused 

 and consisted in destroying, by pricking with a heated needle or 

 the galvanocauter, the right (presumable) optic pit (foveola 

 optica). 1 As a result he found that no lens was formed, if the 

 entire optic pit had been destroyed or, if deeply buried remnants 

 of the latter which developed into diminutive optic cups failed 

 to reach the epidermis. If, however, such a rudimentary optic 

 cup did come into contact with the epidermis, a lens developed 

 secondarily from the latter. 



I am inclined to think that these results practically solved the 

 problem, and for this reason it may perhaps be regarded as un- 

 fortunate that Spemann's well-warranted generalizations have 

 been the subject of ill-founded criticism by Mencl ('03). For 

 w-hile the latter was very aptly met by Spemann ('03), it ap- 



1 For a description of this earliest discernible eye primordium cf. Eycleshymer 

 ('93 and '95) and Froriep ('06). 



