THE LENS-PROBLEM. 227 



the operation, the otic labyrinth came to lie between the epider- 

 mis and the eye, the latter lacked a lens. No lens, however, 

 developed in any of these embryos caudally from the eye, as 

 might have been expected, had that part of the epidermis ("pri- 

 mare Linsenbildungszellen ") been capable of giving rise to a 

 lens by self-differentiation. 



These cases (although so few in number) point decidedly, I 

 think, to the inability of supra-ocular epidermis in R. esculenta 

 to differentiate a lens in the absence of a stimulus from the eye 

 vesicle. Spemann, however, prefers the tentative explanation 

 "dass nach Auslosung einer Linsenbildung durch's Auge die 

 spontane Entwicklung einer zweiten Linse aus den primaren 

 Linsenbildungzsellen unterbleibt." He is inclined to consider 

 even the possibility "dass bei diesen Experimenten die Lin- 

 senbildungszellen nicht weit genug nach hinten gebracht worden 

 waren und dass entweder sie selbst oder ihre nachste Umgebung 

 die Linse des stehengebliebenen Auges geliefert haben." As 

 support for the latter possibility he regards the observation, 

 "dass die Linse in alien 3 Fallen im hintersten Winkel des etwas 

 deformierten Augenbechers liegt und einmal sogar etwas in die 

 Lange gezogen ist" ('12, p. 64). Is it not more probable that 

 this distorted relation between the lens and the optic cup is due 

 to the very fact that the latter was deformed owing to the opera- 

 tion? From my observations on many teratophthalmic Fundu- 

 lus embryos I know that this is very often the case in deformed 

 eyes. 



One cannot escape the impression that Spemann by con- 

 sidering such possibilities and by regarding "diese Versuche . . . 

 als misslungen und die Frage als unentschieden " has apparently 

 unconsciously missed the only obvious conclusion, viz., that 

 Rana esculenta possesses no early predetermined lens-forming cells 

 and that a stimulus from an optic cup or from some of its parts is 

 necessary to induce the differentiation of a lens from the epidermis 

 in embryos of this species as much as in Rana fusca. 



Let us now consider the second (b) series of these experiments 

 in which, as will be recalled, a fragment (the tip, "Kuppe") of 

 the optic vesicle was left attached to the inverted epidermis. 



Observations were made on seven embryos operated upon in 



