226 E. I. WERBER. 



to the optic vesicle owing to faulty technique in the removal of 

 the supra-ocular epidermis. 



Might we not with at least just as much justification assume 

 that in the embryos in which no lenses were formed from the 

 transplanted epidermis this was due to lack of immediate contact 

 between the optic vesicle and the epidermis, because mesodermal 

 cells were attached to the latter? For, regardless of the most 

 painstaking care I think it is impossible to be sure that the 

 epidermis has been cleaned of these cells entirely with the aid of 

 such low magnifications as even the highest power lenses of the 

 binocular dissecting microscope. It is, besides, difficult to under- 

 stand why the optic vesicle of this species should be able to induce 

 lens-formation from ventro-abdominal epidermis of Bombinator 

 and not from the abdominal epidermis of an embryo of its own 

 species. 



Whatever one might think of the results of these experiments, 

 they certainly cannot be regarded as conclusive in favor of Spe- 

 mann's present views. 



Two more series of experiments on Rana esculenta may now be 

 considered the results of which Spemann interprets in favor of the 

 independent development of the lens in this species. The method 

 employed in these experiments was autoplastic transplantation 

 and the part transplanted was the "primaren Linsenbildungs- 

 zellen." 



In the first (a) series a more or less rectangular piece of supra- 

 ocular epidermis was detached during or immediately after the 

 closure of the medullary tube and turned about 180, care being 

 taken that no fragments of the optic vesicle be left on it. In the 

 second (b) series the operation differed only in one point, viz., 

 that in detaching the supra-ocular epidermis a fragment of the 

 optic vesicle remained attached to it and owing to the inversion 

 of the piece of epidermis (by turning it about 180), it became 

 transplanted into a posterior region (otic capsule). 



In the experiments of the first (a) series great technical diffi- 

 culty was encountered and only in four cases was the separation 

 of the epidermis from the optic vesicle "anscheined einwands- 

 frei." In three of these cases the eye of the operated side had 

 "eine deutliche Linse," while in the fourth case where, owing to 



