THE LENS-PROBLEM. 225 



regions of the epidermis? I am under the impression that the 

 apparent fallacy of Spemann's conclusions from this series of 

 experiments is most likely due to the circumstance that while he 

 detected an important source of error, he unfortunately, mistook 

 .the nature of the error. For had he considered the possibility 

 that fragments of the optic vesicle too small to be detected with 

 the binocular dissecting microscope and even so minute as not 

 to be able to differentiate into histologically discernible struc- 

 tures, may suffice to induce the lentogenic reaction in the epider- 

 mis to which they remained attached, he would probably have 

 considered the "unsuccessful" experiments of this series as suc- 

 cessful and vice versa. Accordingly, instead of believing that in 

 the two "successful" cases the "lens-forming cells" had not 

 (or not entirely) been removed, he might have concluded that 

 owing to the above-mentioned difficulty some remnants of optic 

 substance have stimulated the development of the lens-buds 

 demonstrated in Figs. 37, 38 and 38a. 



The results of other experiments performed by Spemann ('12) 

 which may now be examined would also seem to lend support to 

 his former views rather than to his present ones. One of these 

 experiments consisted in transplanting abdominal epidermis 

 over the bared optic vesicle in Rana esculenta. The flap of 

 epidermis used for the purpose was previous to its transplan- 

 tation "von etwa anhaftenden Mesodermzellen sorgfaltig gerei- 

 nigt." The result was negative no lenses or lens-like structures 

 were formed by the strange epidermis owing, as Spemann thinks, 

 to the circumstance that the "primaren Linsenbildungszellen " 

 were contained in the flap of supra-ocular epidermis which had 

 been removed. However, four cases were recorded in which the 

 epidermis transplanted over the eye presented an appearance 

 different from that of the epidermis of the immediate surround- 

 ings. And in Fig. 63 there is "eine deutliche Linsenanlage (L) 

 vorhanden, ein kleines, dickwandiges Epithelblaschen, welches 

 mit der transplantierten Rumpfhaut iiber dem Auge innig 

 verbunden ist." ('12, p. 55). Spemann mentions also another 

 embryo of this series in which he observed an indication of lens- 

 formation on the side operated upon (Fig. 58), but assumes that 

 in both cases "primare Linsenbildungszellen" remained attached 



