ON THE BLASTOLYTIC ORIGIN OF THE 'INDEPEND- 

 ENT' LENSES OF SOME TERATOPHTHALMIC EM- 

 BRYOS AND ITS SIGNIFICANCE FOR THE NORMAL 

 DEVELOPMENT OF THE LENS IN VERTEBRATES^ 



E. 1. WERBER 



Osborn Zoological Laboratory, Yale University 



TWO TEXT FIGURES AND TWO PLATES 



The problem of the development and differentiation — depend- 

 ent or independent — of the lens of the vertebrate eye has in 

 recent years formed the subject of many interesting investiga- 

 tions. 



In 1901 Spemann attempted its solution by experiments in 

 which the anlage of an optic cup in the neurula-stage of Rana 

 fusca was mechanically destroyed. From the failure of many 

 larvae to develop a lens on the side operated upon he concluded 

 that the lens of the vertebrate eye depends for its development 

 upon a contact stimulus from the optic cup. Herbst ('01) con- 

 cluded from the fact that in higher vertebrates the development 

 of the lens never begins before the optic vesicle has come into 

 contact with the ectoderm from which it arises that the latter 

 is essential and that the development of the lens is to be regarded 

 as a 'specific' (chemical?) 'thigmomorphosis.' As evidence for 

 the justification of this hypothesis he adduced the cyclopean 

 monsters, i.e., embryos with a single median eye. In such em- 

 bryos, he reasoned, no lenses develop on the sides of the head, 

 that is, in the usual position of the eyes, because the ectoderm 

 of these areas, owing to the absence of lateral optic vesicles, 

 does not receive the specific thigmomorphotic stimulus neces- 

 sary for its differentiation into lenses. On the other hand, he 

 concluded, since a lens does develop in association with the 



^ Aided by a grant from the Carnegie Institution of Washington. 



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