348 E. I. WERBER 



median eye from ectoderm which normally does not give rise 

 to a lens, it is evident that the heterotopic (cyclopean) optic 

 vesicle furnishes the formative stimulus for its differentiation 

 from indifferent ectoderm. 



Soon, however, the validity of this exceedingly suggestive hy- 

 pothesis, became questionable, for Mencl ('03) described two 

 lateral lenses in an anophthalmic head (without any traces of 

 optic cups) of an anadidymus in Salmo salar. From these find- 

 ings he concluded that no stimulus from the optic cup was nec- 

 essary for the differentiation of the lens. More recently Mencl 

 ('08) has recorded free lenses in several anophthalmic Salmo 

 embryos. 



In the meantime the problem had again been attacked ex- 

 perimentally by Barfurth ('02) W. H. Lewis ('04), King ('05), 

 Le Cron ('07) and Spemann ('07). The evidence which Bar- 

 furth was able to bring forward for the 'dependent' differen- 

 tiation of the lens in the chick was rather inconclusive. Of a 

 very decisive character, however, seemed to be the results which 

 Lewis obtained in his experiments on Rana palustris. He re- 

 moved the optic vesicle of tadpoles with the least possible injury 

 to the overlying ectoderm which was raised from it and reflected 

 forward. No lenses developed in such embryos on the side op- 

 erated on and Lewis' conclusions fully confirm Herbst's views. 

 Only a year later, however, King experimenting on embryos of 

 the same species by destroying the optic vesicle with, a heated 

 needle, was able to show that lens buds developed on the eye- 

 less side of the embryo which might perhaps have fully differ- 

 entiated into lenses, had the embryos been kept alive for a suf- 

 ficient length of time. This remarkable discrepancy of results 

 increased from now on with practically every attempt at the 

 solution of the problem. Le Cron, experimenting on Ambly- 

 stoma was able to confirm Lewis' important findings, but Spe- 

 mann ('07) found that in Rana esculenta a lens may be formed 

 without any stimulus from an optic vesicle or cup. In the next 

 year Mencl was able to confirm the observations he had recorded 

 in 1903, and in 1909 Stockard employing a chemical method, suc- 

 ceeded in producing in Fundulus experimentally many terato- 



