146 Lens-Formation from Strange Ectoderm 
and that contact between optic vesicle and ectoderm is also necessary as 
the optic vesicle evidently has no power of acting at a distance to start 
lens-formation. Moreover, not only contact but actual adhesion be- 
tween optic vesicle and ectoderm seem necessary. Such adhesion always 
takes place in the course of normal Jens-formation. The size of the 
early lens structure, the lens plate, the lens bud, and the lens vesicle, is 
dependent in part at least upon this area of contact or adhesion between 
optic vesicle and ectoderm and in part also upon the length of time the 
optic vesicle or optic cup remains in contact by its retinal layer with 
the developing lens. 
ANATOMY OF THE OPERATING STAGE AND METHOD OF OPERATION. 
The conditions in rana sylvatica and rana palustris are essentially the 
same at this early stage, namely, at the time of, or shortly after, the 
closure of the neural folds. The anatomy of the eye region I have 
already described.” The optic vesicle projects from the ventral one-half 
of the lateral surface of the brain and is in contact with the ectoderm 
but not adherent to it. There are, of course, no signs of lens-formation 
nor is the optic vesicle adherent to the ectoderm. ‘The optic vesicle was 
transplanted into the otic region in each embryo (Fig. 1). 
The embryos were operated upon under a binocular microscope. An 
incision was made caudal to the bulge formed by the optic vesicle, a 
skin flap turned forward from over the eye and the optic vesicle cut off 
with a sharp needle or a fine pair of scissors (see Fig. 3a Am. Jour. of 
Anat. Vol. VI, p. 493). From the caudal edge of the incision a pocket 
was made beneath the ectoderm of the otic region. In this region (Fig. 
1) at this stage there is a thickening of the inner layer of the ectoderm 
forming the rudiment of the otic vesicle. In pushing the needle back 
and forth to make the pocket between ectoderm and mesenchyme the 
ectoderm was often injured over the pocket and also at the edge of the 
pocket, and even pieces of the ectoderm were detached and left in the 
wound and later carried into the mesenchyme with the eye. After for- 
mation of the pocket the optic vesicle was pushed into it with a blunt 
needle. Sometimes it slipped in quickly and without much injury, often, 
however, it must have been torn and distorted. The mesenchyme was 
always more or less injured and often some of it taken out to make room 
for the large optic vesicle, as one can see from Fig. 1 there is not much 
space between ectoderm and entoderm. In most of the embryos at the 
Am. Jour. of Anat., Vol. VI, p. 474, and figures 1, 2, and 3. 
