THE LEXS-PROBLEM. 221 



patently occasioned him to enter upon a great many experiments 

 on the subject which, though beautifully conceived and skillfully 

 executed, have, owing to the pitfalls of a treacherous material, 

 yielded fallacious results. 



The first experiments which have occasioned Spemann ('07) 

 to modify his views were performed on Rana esculenta. They 

 consisted in the excision of the "right anterior half of the brain 

 primordium" from the wide open medullary plate. 1 As a result 

 he observed that a lens has developed in spite of the absence of an 

 eye on the side operated upon. Like results were obtained also 

 when the optic pit was destroyed at the same stage of develop- 

 ment with a heated needle. The results were apparently entirely 

 independent of the method of operation. In both cases, however, 

 they differed from those obtained in Rana fusca, in which, as we 

 have seen, no lens was formed when the optic anlage was at this 

 stage destroyed by pricking with a heated needle, while the 

 excision experiment in this species, according to Spemann ('07) 

 "ergab ein sehr unsicheres Resultat." In one out of four ex- 

 amined embryos Spemann ('12) observed "ein kleines Blaschen, 

 welches eine Linsenanlage sein konnte." 



Why such divergent results in experiments on two species of 

 the same frog genus with the same methods and at the same stage 

 of development? Spemann concludes from his experiments on 

 Rana esculenta that in this species the "lens-forming cells" of the 

 embryonic epidermis are capable of independent development in 

 the absence of an optic vesicle, while Rana fusca lacks this ability 

 for self-differentiation. 



The same experiments on Bombinator pachypus (at the same 

 stage of development) gave less definite results. While no lens 

 developed in this species on the side operated upon, Spemann 

 observed in some few instances structures which he could not 

 with certainty identify. Experiments on embryos of the same 

 species at a later stage of development, i. e., removal of the optic 

 vesicle, the overlying epidermis being previously raised up and 

 reflected from it, gave also ill-pronounced results. In a number 



1 In my 19160 paper several, otherwise wholLy insignificant, errors have unfor- 

 tunately crept in into the references made to these excision experiments in Rana 

 esculenta and to King's pricking experiments. In both cases it should read 

 "optic pit" or "optic anlage" instead of "optic vesicle." 



