222 E. I. WERBER. 



of instances Spemann observed in such embryos on the side 

 operated upon a thickening of the epidermis in the region where 

 a lens normally should have developed, which he is inclined to 

 consider as an abortive attempt at lens-formation. From these 

 experiments Spemann tentatively concludes that Bombinator in 

 regard to the power of independent lens-formation occupies a 

 sort of intermediate position between Rana fusca which lacks 

 this ability and Rana esculenta which, according to him, possesses 

 it in a decidedly marked degree. 



Are these conclusions justified? I am strongly inclined to 

 think that they are not, the results on which they are based 

 being very inconclusive. Moreover, on careful examination of 

 the excision experiments in Rana esculenta the impression is 

 gained that some fragments of optic substance w r ere left after the 

 excision of the anterior half of the primordium of the brain hemi- 

 sphere. Thus, for instance, in his 1912 paper in Fig. 6 the smaller 

 one of the two observed lentoids is in close apposition to a "gang- 

 lion." May not this ganglion rather be a small group of retinal 

 cells? In Figs. 8 and 9 small groups of tapetum nigrum cells 

 may be observed, and Spemann (p. 22) admits "dass vom Augen- 

 becher vielleicht kleine Reste vorhanden seien." In Fig. 8 the 

 tapetum-cells are very close to the lentoid, while Fig. 7 also sug- 

 gests that dispersed tapetum nigrum was responsible for the 

 origin of its "Linsenblaschen." Instead of concluding, as does 

 Spemann, that the lentoids of Figs. 6 and 9 are due to dispersion 

 of "lens-forming cells" it might perhaps be safer to think that in 

 these experiments fragments of potential optic-cup substance 

 were dispersed and stimulated the differentiation of lentoids by 

 chancing to come into contact w r ith the epidermis. 



Again, in Fig. 10 representing a cross-section through an em- 

 bryo in which the anterior part of the anlage of the right brain 

 hemisphere has been destroyed by a heated needle, the lentoid 

 observed is in close proximity to a rudimentary optic cup (oc', 

 Fig. 10), which clearly demonstrates the fact that the attempted 

 elimination of the anterior part of the brain primordium may 

 often be illusory, fragments of optic-pit substance remaining 

 after the operation. According to Spemann ('12, pp. 24 and 25) 

 it is easier to eliminate the entire optic anlage in Rana fusca than 



