ORIGIN OF 'independent' LENSES 349 



phthalmic monsters in which the free lens was of rather frequent 

 occurrence. From his observations which strongly corroborate 

 Mend's statements, Stockard concluded that the lens is capable 

 of development without any stimulus from the optic cup. 



Mend's and Stockard's results and their conclusions seemed to 

 be very convincing, and, on the whole, the major evidence seemed 

 to be in favor of the independent origin of the lens by 'self-dif- 

 ferentiation.' The climax of the difficulty, however, had not yet 

 been reached, for, in 1912 Spemann reported on the basis of very 

 extensive experiments that the same methods of operation on the 

 same stages of frog larvae of two species belonging to the same 

 genus yielded conflicting results, the one species being capable 

 of forming lenses without the stimulus of an optic cup, while 

 in the other species lenses were not formed if the optic vesicle 

 had been removed. 



What is the cause of this remarkable incongruity of appar- 

 ently correct observations? Why is Amblystoma incapable of 

 forming a lens independently of the optic cup, while in Rana 

 esculenta this ability apparently exists? Why is Rana palustris 

 capable of differentiating a lens without the contact stimulus 

 from an optic cup, when the optic vesicle is destroyed by a heated 

 needle and incapable of doing so when the latter is removed by 

 cutting? Why in cyclopean Fundulus embryos does the single 

 median optic vesicle stimulate by contact the formation of a lens 

 from ectoderm which would normally not differentiate into a 

 lens, while in other teratophthalmic, and particularly, anophthal- 

 mic embryos of the same species free lelises frequently occur with- 

 out any trace of an optic cup? And, finally, why should two 

 species of the same genus differ so widely in their morphoge- 

 netic potentiality, the one (Rana esculenta) being capable of form- 

 ing a lens when the optic vesicle has been removed, while in the 

 other (Rana fusca) this apparently does not occur, after the 

 same operation performed at the same larval stage? 



The answer to these perplexing questions, I think, is that we 

 are here dealing with an apparently morphological problem for 

 whose solution neither our morphological methods of investiga- 

 tion nor mechanical methods of experimentation seem to be 



