THE LENS-PROBLEM. 237 



This author transplanted in well-advanced larvae (hind legs 

 developed) of Triton tceniatus, in which the eye was at the time of 

 operation as fully developed as in the adult, a piece of the iris 

 from the eye of one larva into the otic capsule of another larva 

 of the same age. As a result he could in one case observe that a 

 strand of epidermal epithelium which, owing to the operation, 

 had grown in from the edge of the wound to the otic capsule, has 

 given rise to "ein rundes Lentoid . . . mit konzentrisch gelager- 

 ten Linsenfasern . . .." 



From all that has been said so far we may conclude that there 

 exists no valid evidence for the possibility of the origin of the lens 

 by self-differentiation. On the contrary, observation in the normal 

 development of the eye, evidence from teratophthalmic cases and all 

 experimental evidence point to the correctness of Herbst's ( '01) and 

 Spemann's ('o/) conclusion that the lens of the vertebrate eye de- 

 pends in its development and differentiation from ectodermal epi- 

 thelium upon a specific, apparently chemical, stimulus from the 

 optic vesicle. 



This stimulus might perhaps be in the nature of an enzyme 

 action by contact. Spemann ('05, '12), who recognizes the 

 dependence of the development of the lens upon this specific 

 stimulus in some species, assumes the possibility of a specific 

 secretion by the optic cup or rather by the retinal part of the 

 latter, to which, as he believes, may also be due the "regenera- 

 tion" of the lens from the iris of the fully developed amphibian 

 eye. While this hypothesis appears to have received strong 

 support from the beautiful experiments of his pupil Wachs 

 (/. c.), it seems to me perhaps premature to speak of a secretion 

 in this case. For, aside from other considerations, the fact 

 ascertained by Wachs (/. c.} that the supposedly secreted sub- 

 stance is not conveyed by the blood of the animal to any other 

 part of the body would seem sufficient to indicate that the retina, 

 as Wachs himself concludes, has no such endocrine function. 

 Nor is Wachs 's assumption of a secretion that "bleibt auf das 

 Auge und seine nachste Umgebung beschrankt" (p. 446) justified 

 by the results of his numerous experiments. 



There is, moreover, good reason to believe that the capacity 

 for the "lentogenic stimulus" is not at all restricted to the retinal 



