266 The Origin and Differentiation of the Optic Vesicle 
formation of the internal membrane-like structure and instead of direct- 
ing the axis cylinders, as under normal conditions, towards the cleft 
and optic stalk they passed through the membrane into the cup cavity 
and so out through the pupil into the mesenchyme. By other disturb- 
ances in this membrane or by a change in the orientation of the ganglion 
cells we can account in a general way for the nerve penetrating the layers 
of the retina at various places, and by a variable path of least resistance 
the various paths of the nerves beyond the retina or in the pigment 
layer. 
IT should not wish to leave the impression that the eye possesses no, or 
very little, regulative power, but rather that a distortion in transplanting 
may readily be so extensive that a perfect eye cannot form. 
Several eyes can probably develop from a single optic vesicle; Fig. 13 
shows a large transplanted one, Fig. 15 a small transplanted one, and 
Fig. 14 the regenerating eye, all originally from the same optic vesicle. 
The total volume of the transplanted and regenerated eve from the 
same original optic vesicle may be nearly twice as great as the normal 
one, indicating that both transplanted and regenerating eyes possess 
considerable power of regeneration. 
ON THE REGENERATION OF THE EYE. 
The right eye was cut away in 350 embryos, 180 of rana palustris and 
170 of rana sylvatica, and the embryos were killed from 2 to 20 days 
after the operation. The optic vesicle was cut away, as we have already 
noted, at about the stage of closure of the neural folds and in most of the 
experiments transplanted into the same or another embryo. 
The results as regards the regeneration of the eye from these opera- 
tions are very varied but are all readily explained on the assumption, Ist, 
that those cells which go to form the eve are already determined and 
that the line of separation between brain and eve cells is fairly sharp and 
can be indicated by a plane having a position somewhat as the hne cd, 
Fig. 3a." 2d, That at this stage regeneration of the eye can only take 
place from these cells which are already determined, and that the brain 
is not capable of regenerating an eye. 
In a number of the embryos killed even as late as 20 days after the 
operation there is no trace of a regenerated eye. Some of these show a 
perfect bilateral symmetrical brain, and a transplanted eye often as 
large or larger than normal which may or may not have a small piece 
1* Lewis, Am. Jour. of Anat., Vol. VI, 1907, p. 493. 
