224 E - ! WERBER. 



to "ein deutliches Linsenblaschen " in the absence of the optic 

 cup. Spemann considers these lens-buds as evidence of the 

 ability for independent lens formation in R. esculenta. 



The validity of this conclusion, however, seems questionable 

 to me in view of a well-nigh uncontrollable source of error which 

 Spemann himself has pointed out ('12, p. 42). For he states 

 expressly that the lower stratum of the epidermis, (the "Sin- 

 nesschicht") adheres so firmly to the eye vesicle that on attemp- 

 ting to separate the epidermis from the latter usually the upper 

 layer (the "Deckschicht") is raised up, while the lower one re- 

 mains attached to the optic vesicle. Regardless of Spemann 's 

 skill in such delicate operations and the precautions he has taken 

 to separate the entire epidermis from the optic vesicle it was 

 apparently impossible always to avoid minute fragments of the 

 latter remaining attached to it. And the two cases in which he 

 obtained "independent" lens-buds have possibly resulted from 

 just such an unsuccessful operation, while in the embryos in 

 which no such structures were recorded on the side operated 

 upon the operation was apparently faultless. 



Spemann assumes that in his unsuccessful experiments of this 

 series, i. e., where no "independent lenses" were recorded, the 

 "lens-forming cells" of the lower stratum of the epidermis were 

 removed with the optic vesicle to which they remained attached. 

 I can see no valid reason for the assumption of cells predetermined 

 to form lenses. The often-raised argument of the cyclopean eye 

 in which the lens is formed from epidermis that normally does 

 not develop into this structure, and Lewis's ('04, 'oya and b) 

 experiments in which the formation of lenses from strange ecto- 

 derm and even from ectoderm of another species was demon- 

 strated to be possible- an experiment successfully repeated by 

 Spemann himself by transplanting the bared optic vesicle of 

 Rana esculenta under the ventro-abdominal epidermis of Bom- 

 binator speak very decidedly against this assumption. 1 



Moreover, why should just the species Rana esculenta form 

 such a strange exception as to possess such so early specialized 



1 Mencl ('08, Fig. 3) described an " intracerebral eye" (intracerebral cyclopia) 

 whose lens is derived from the epithelium of the mouth. The ability of this part 

 of the ectoderm to give the "lentogenic reaction" is demonstrated also by the 

 lentoids of the mouth which I have recorded (Werber 'i6c). 



