248 Origin and Differentiation of the Lens 
lens, farther than the other embryos of this series. That this 
is possible without any indication on the surface is very 
probable. In the examination of a number of embryos of Rana palustris, 
it was noted that embryos, which so far as external features were con- 
cerned seemed of the same age, often differed considerably in the amount 
of differentiation of the lenses. If this is true for Rana palustris it 
probably holds also for Amblystoma. Consequently some variations 
must be expected in the results obtained from embryos which are alike 
so far as their external features are concerned. And so, I am inclined to 
believe that the rudimentary lens-bud in the above experiment indicates 
that the optic vesicle had already exerted some influence on the ectoderm 
leading to lens formation, but when this influence was removed those 
ectodermal cells possessed but very little power of self-differentiation, 
and hence the development of the lens soon came to a standstill. That 
this is true will become more evident when we consider the results ob- 
tained after removal of the optic vesicle at later stages. 
The foregoing experiments indicate very clearly then, that the lens 
is not a self-originating structure, and that it is dependent for its origin 
upon the influence or stimulus of the optic vesicle. If the optic vesicle 
is necessary for the starting of the lens, it may also be necessary for its 
differentiation, and if so, the removal of the influence of the optic vesicle 
during the various stages of lens development, should be followd by re- 
tardation and abnormal growth of the lens. 
The optic vesicle was next removed from embryos at a stage (VIII) 
slightly older than those first operated upon. At this stage the tail-bud 
is just beginning to show and in most of the embryos the lens-plate ap- 
pears as a very slight thickening of the inner layer of the ectoderm, 
where the optic vesicle comes into contact with it (Figs. 4 and 5). | 
Three embryos (Experiments VIIT,, ,, ,) thus operated upon and 
killed two, five and twelve days afterwards, were without regenerated 
eyes on the right sides and without traces of lens formation. On the 
normal or left side of the embryo killed two days after the operation 
there is a marked thickening of the ectoderm forming the lens-plate. 
The five-day embryo shows on the left or normal side a well-formed Jens 
separated from the ectoderm and with the beginning of the formation 
of lens fibers. This normal lens is about 130 in diameter. The em- 
bryo that was allowed to live 12 days has on the normal side a well dif- 
ferentiated lens, about 190 in diameter, with long lens-fibers. Al- 
though the external appearances of these embryos would have indicated 
that the lens-plate had begun to form, nevertheless such thickenings on 
the under side of the skin flap were not observed at the time of the op- 
