THE LENS-PROBLEM. 243 



ciple similar to the conditions obtaining in the primary (embry- 

 onic) formation of the lens, although the morphogenetic scheme 

 of the latter cannot be fully applied to either the retinal lentoids 

 or to the lens formed from the iris. The chief difference between 

 the two modes of the morphogenesis of the lens is that while the 

 origin of the "lentogenic stimulus" and the part responding to 

 it are locally separate in the primary formation of the lens, they 

 would granting the correctness of our interpretation coincide 

 locally in the secondary formation of the lens from the iris or of a 

 lentoid from the retina. 



In order to fully understand the morphogenetic process under- 

 lying the "regeneration" of the lens from the iris (or of lentoids 

 from the retina), it would also seem necessary to explain just how 

 the "lentogenic stimulus," the latent capacity for which is prob- 

 ably possessed by the whole optic cup proper, is activated on the 

 extraction of the primary lens. An answer to this query is, I 

 believe, partly given by some of Fischel's ('02) experiments, 

 which I shall review in the following. 



The lens is so tightly enclosed by the iris that on its extraction 

 slight injuries to the latter result. No matter how carefully the 

 operation may be performed, slight, unnoticeable, lesions (abra- 

 sions?) of the cells of the pupillary edge of the iris may be una- 

 voidable. 1 The significance of these, otherwise perhaps negli- 

 gible, physical alterations of these cells of the iris resulting from 

 the operation suggested itself to Fischel ('02, pp. 106-109 ff) 

 from the observation of small lenses and lentoids formed not only 

 by the pupillary edge but also from other parts of the iris. Being 

 aware of accidental injuries to the eye cup and to the iris in some 

 of his experiments he thought of a possible causal connection be- 

 tween the latter and such lenses or lentoids located in various 

 parts of the iris. In order to test the correctness of his tentative 

 interpretation he performed a number of experiments in which 

 besides the exstirpation of the lens various parts of the iris were 

 purposely injured. On microscopic examination of the results 

 he was able to observe that small lenses or lentoids were formed 



1 According to E. Uhlenhuth (quoted from Loeb '16) who cultivated fragments 

 of the iris by Harrison's explantation method, it is to these abrasions that the loss 

 of the pigment by the cells of the iris is due. Owing to this injury of the cells 

 the pigment granules are liberated into the lumen of the optic cup where, as Wolff 

 ('95) has observed, they may be absorbed by leucocytes. 



