THE LENS-PROBLEM. 239 



mation of the lens from the epithelium of the iris is probably 

 a response (reaction or complex of reactions) to a chemical stim- 

 ulus just as much as its primary development from the supra- 

 ocular epidermis of the early embryo. 



But, it may well be asked, if there is no such secretion as as- 

 sumed by Wachs, what is the nature of the chemical stimulus 

 and whence, from what source, or from what part of the eye does 

 it issue? Two possibilities suggest themselves as an answer to 

 this query. It may be imagined that, owing to the operation, 

 the iris comes into temporary contact with the inner wall of the 

 optic cup, from which it might thus receive the stimulus for the 

 formation of a lens in the same manner as in the embryo the supra- 

 ocular epidermis from the optic vesicle. Such contact due to a 

 collapse of the optic cup 1 may result from the methods of opera- 

 tion employed by Wolff ( '95) and the other experimenters (with 

 the exception of Wachs). However, I am not inclined to give 

 serious consideration to this possibility, as this contact of the 

 pupillary edge of the iris is not necessary for the "regeneration" 

 of a lens, for in Wachs 's experiments the method of operation 

 excludes it altogether. The experiments of the latter author, 

 however, do not, as he believes, exclude another possibility which 

 to me seems well worthy of careful consideration. 



Wachs (/. c., pp. 416-426) has raised the question whether the 

 iris is capable of forming a lens without any outside stimulus 

 ("aus sich heraus"). He was led to consider this possibility 

 by the results of some of his experiments, in which a fragment of 

 the iris, implanted into the posterior chamber of an eye deprived 

 of its lens "regenerated" a lens, even if it did not heal on to the 

 iris. This lens may even be better differentiated than the lens 

 simultaneously "regenerated" by the latter. 



To decide whether the fragment of iris in these cases formed a 

 lens under the influence of a secretion from the retina or- "aus 



1 According to a statement by M tiller ('96) no lens is formed by the iris aftei 

 extraction of the primary lens, if the vitreous body is injured. Were this state- 

 ment correct, no lens could arise from the iris owing to its contact with the optic 

 cup by collapse of the latter. It is evident, however, that Miiller's observation is 

 incorrect, for in Wolff's (/. c.) and in some of Fischel's ('oo and '02) experiments 

 in which a lens or lentoids did form from the iris, the vitreous body was undoubtedly 

 injured, since the whole optic cup was damaged. 



