476 The Origin and Differentiation of the Lens 
leading to lens-formation, nor do these changes appear for some little 
time. That is, no indications are to be seen by the ordinary histological 
and embryological methods. The cells of the inner layer of the ectoderm 
are apparently all alike in this region. That there may be invisible 
differences in the cytoplasm or in the nuclei, or differences which have 
not as yet been recognized, is, of course, possible, but the following ex- 
periments would seem to indicate otherwise. 
Thus the optic vesicle will continue its differentiation independently 
of any especial environment, as when transplanted into various regions 
of the embryo. Not, so, however, does the normal lens-forming ecto- 
derm behave when its special environment is altered by removal of the 
optie vesicle, for without the influence of the optic vesicle no traces 
of the lens appear. 
If the lens-forming ectodermal cells are different—the difference only 
appearing as development progresses—we should expect this to appear 
independently of their special environment (the presence of the optic 
vesicle) unless this environment is a necessary factor in bringing about 
their progressive differentiation, and as such environment is necessary 
the lens cannot be considered as a self-originating structure. It is 
only by the experimental method that we can so alter the normal 
environment as to afford a means of solving such questions. The 
elimination of the possible influence of the optic vesicle is naturally 
the first factor to be considered. | 
Method of Operation. 
The embryos were operated upon under the binocular microscope. 
They were placed in small glass dishes either in ordinary tap water 
or in a 0.2 per cent salt solution. The latter solution does not offer 
any especial advantages, however. The embryo was held with a small 
pair of forceps and a semi-circular incision made, through the ectoderm 
caudal in the bulge produced by the optic vesicle, with a fine pair of 
scissors or a sharp needle. The skin flap thus formed was turned for- 
ward and left attached anterior to the eye. The optic vesicle and 
surrounding structures were thus exposed without injury either to them 
or the overlying ectoderm. In Fig. 2 a much larger skin flap than 
was used in the operation is shown turned forward from over the optic 
vesicle and ganglionic masses. At a later stage the operation of turning 
forward the skin flap becomes quite difficult, owing to adhesion which 
takes place between optic vesicle and ectoderm preceding lens-formation. 
