THE LENS-PROBLEM. 23! 



On transplanting ventro-abdominal epidermis over the bared 

 optic vesicle in this species Spemann recorded as a result that in 

 none of the sixteen examined embryos a lens developed from the 

 strange epidermis. From these results he concludes that either 

 the optic vesicle cannot stimulate abdominal epidermis to the 

 formation of a lens or that this part of the epidermis is incapable 

 of responding to such a stimulus. 



Here again, however, the same objection is unavoidable which 

 we have raised against the conclusion from the results of the same 

 experiments in Ran a esculenta, namely that regardless of the 

 experimenter's care mesenchyme cells attached to the trans- 

 planted epidermis obstructed a contact of the latter with the 

 optic vesicle. 1 



Very clear ("eindeutig") results were obtained in the experi- 

 ments in which, as in Rana esculenta, a flap of the skin of the 

 head including the supra-ocular epidermis was turned about 

 180. In the first (a) series in four out of eight "einwandsfreie" 

 cases the eye cup was not in contact with the epidermis and, 

 naturally, lacked a lens. In the other four cases there was such 

 contact and the eye possessed a lens. 



Accordingly : There are no lens-forming cells, but epidermis that 

 normally does not give rise to a lens will differentiate into a lens, if 

 brought into contact with the optic vesicle. 



The second (b) series of these experiments (a fragment, the 

 "Kuppe," of the optic vesicle transplanted with the turned 

 epidermis) yielded results very similar to those of the corres- 

 ponding experiments in Rana esculenta. 



With but one exception the posterior eye fragment always 

 obtained a lens, which, as Spemann rightly adds, could not have 

 developed in the absence of this fragment of optic-cup substance. 



The anterior eye fragment, however, obtained no lens in fifteen 

 out of the twenty examined embryos, while a lens or only a 

 thickening of the epidermis opposite the eye fragment was ob- 



1 It is significant that in one out of the four cases in which he reports to have 

 observed a "Wucherung" opposite the eye "eine zusammenhangende Schicht sehr 

 dotterreicher Zellen" (p. 60) was conspicuous in that interspace. That shows that 

 it is apparently futile to attempt to clean the abdominal epidermis of all mesenchyme 

 cells. While it is difficult to know just how to interpret these " Wucherungen," 

 they might possibly be due to mechanical distortion during the operation. 



