THE LENS-PROBLEM. 245 



ontogeny. Already at that time he regarded it as a primary 

 potency of all derivatives of the ectoderm. As the additional 

 factor, however, necessary for this transformation of ectodermal 

 derivatives he assumed an unknown stimulus which in the 

 "regeneration" of the lens from the iris "in jenen Alterationen 

 zu suchen ist, welche das regenerierende Gewebe direkt durch den 

 experimentellen Ein griff selbst erfdhrt . . . von welchen die 

 Zellen der Iris bei der Linsenextraktion betroffen werden" 

 ('02, p. 106). 



This unknown stimulus Fischel ( '16) now regards as a chemical 

 one from a secretion by the optic cup, or more specifically, from 

 the retina which in conformance with Spemann and Wachs he 

 also assumes. To the stimulus from this secretion he attributes 

 the transformation of the supra-ocular epidermis into a lens in the 

 embryo as well as the like transformation of the cells of any in- 

 jured part of the optic cup (iris or retina). 



While in the main I agree with Fischel by recognizing the 

 necessity of an injury to that part of the iris or retina from which 

 the secondary lens or lentoid arises, as proven, I am, as already 

 stated, inclined to doubt the probability of a secretion by the eye 

 cup as the additional factor involved in the stimulus for the for- 

 mation of the secondary lens. Undoubtedly, with the present 

 data at hand, it is difficult to deny that some chemical process is 

 apparent in the morphogenesis of both the primary and the secon- 

 dary lens. This process, however, may, as was pointed out in the 

 preceding pages, be due to a substance (a "lentogenic enzyme") 

 apparently contained by all parts of the eye cup already in the 

 stage of the optic vesicle or even earlier. The activation, how- 

 ever, of this "enzyme" seems in the case of the "regeneration" 

 of the lens to be in some way due to the injury of those cells of 

 the iris which subsequently become transformed into lens fibers. 



What would yet remain to be explained is the nature of the 

 relation of the injury of the cells to the activation of their own 

 "lentogenic enzyme." On this point, however, I should venture 

 no opinion at the present time. 



One apparent objection to our interpretation may yet be 

 considered. 



It is known from experiments by Wolff ('03) that if a part of 



