Independent Development of the Lens. 415 
is not entirely out of accord with the present facts of lens 
formation. | 
Lewis thinks that the free rudimentary lenses in Schaper’s 
experiments were undoubtedly caused by the shifting of the ecto- 
derm and lens away from the optic vesicle. The writer is unable 
to see.any reason why such a shifting is supposed to have taken 
place. On the contrary, the facts seem to lend themselves more 
readily to the interpretation of a free origin of the lens. 
The experiments of Lewis (’04) seemed to show convincingly 
that the lens was dependent for its origin and development upon 
a contact stimulus of the optic vesicle on the ectoderm. Lewis 
devised a method far more refined thar any previously used in 
similar experiments. He operated on tacpoles under a binocular 
microscope with needles and small scissors and was able to cut 
the ectoderm and fold it forward so as to expose the brain and 
early optic vesicle, which could now be cut away. The ectoderm 
was then folded back inplace. Frcm these experiments the follow- 
ing are some of the conclusions which were drawn: 
Neither a lens nor a trace of a lens will originate from the ecto- 
derm which normally gives rise to one, if the contact of the optic 
vesicle with the skin is prevented. 
There is no predetermined area of the ectoderm which must 
be stimulated in order that a lens may arise. Various parts of 
the skin when stimulated by optic cup contact may and do give 
rise to a lens. 
In normal development the lens is dependent for its origin 
and differentiation on the contact influence or stimulation of the 
optic vesicle on the ectoderm. 
The conclusions are clearly stated and the experimental method 
employed is most skillful, but not entirely free from objection. 
The conclusion regarding the entire dependence of the lens on 
an optic vesicle stimulus will not, the writer believes, be supported 
by future experiments, and indeed at the present time such an 
idea has a vast amount of evidence against it, unless the partic- 
ular species experimented with shows a specific action in lens 
formation. The later work of Spemann (’07) which contradicts his 
former conclusion shows that in some species of frogs the lens 
