Independent Development of the Lens. 413 
lenses, that is, a cyclopean monster with lenses in the usual 
lateral positions. 
After a study of a great number of such monsters I am now able 
to reaffirm that free lenses do occasionally arise in the lateral 
eye regions. Herbst’s clever argument based on pathological 
embryos is, therefore, rendered invalid. 
Spemann (’01) was the first to clearly attack the problem experi- 
mentally. He injured or destroyed the optic vesicles of Triton 
embryos by means of hot needles and electric cauterizers. The 
method was not altogether satisfactory since such an operation 
often injures much of the surrounding tissue, yet Spemann’s 
results were of the highest value, and stimulated an active interest 
on the part of many experimenters. His conclusions furnished 
strong support for Herbst’s idea of the dependent origin of the 
lens. He found that whenever the optic vesicle was so injured 
that it failed to come in contact with the head ectoderm, the 
ectoderm failed to form alens. The lens was, therefore, depend- 
ent for its origin on a contact stimulus of the optic vesicle upon 
the ectoderm. Later Spemann (’05) also concluded that the lens 
was not self differentiating but that a durable contact stimulus 
of the optic cup was necessary for it to form lens fibers. Thus 
the ‘‘Herbst-Spemann theory of dependent lens formation,”’ 
as Mencl has termed it, was developed. We shall see below, 
since it seems best to consider these papers in a more or less chrono- 
logical order, that Spemann’s own later work has helped materi- 
ally to overthrow this theory. 
Barfurth (’02) also operated witha hot needle to destroy the lens 
anlage and the optic cup. The embryos were examined after 
five or six days. One specimen showed on one side a poorly 
regenerated optic cup that did not come in contact with the ecto- 
derm and yet a lens still connected with the ectoderm was present 
on this side. Barfurth was inclined to accept the Herbst-Spe- 
mann idea of lens formation and so attempted to harmonize his 
case with the theory as follows. Some sections of the embryo, 
9 to 12, showed the optic vesicle lying very near the ectoderm 
but not in contact with it. Barfurth supposes that at an earlier 
period in development it may have been in direct contact with 
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