Warren Harmon Lewis 475 
bulging of the ectoderm on the surface of the embryo (Figs. 1, 2, and 3). 
The cavity of the optic vesicle communicates with the brain cavity by 
a wide opening and there is very little indication of the formation of 
the optic stalk. The ganglionic mass of the fifth nerve lies in direct 
contact with the posterior surface of the optic vesicle and partially 
covers it. The walls of the optic vesicle are of about the same thickness 
as the ventral wall of the brain. There is at this time no indication 
of any differentiation between those cells which are destined to form 
the eye, or the optic stalk and the brain. That there are differences 
not brought out by the ordinary histological methods is evident from 
the results of my experiments. Among the embryos allowed to live 
about the same length of time after partial or total extirpation of the 
optic vesicle, there are regenerated eyes of all sizes, ranging from those 
of nearly normal size, to complete absence. In some, only the optic 
stalk has regenerated, and in others the brain wall may be defective 
from loss of tissue, which was cut away with the optic vesicle. Correlated 
with these differences in the regenerated eyes are differences in the 
transplanted eyes. In most of the embryos the optic vesicle, after having 
been cut away, was transplanted into the otic region of the same or 
another embryo, and among these transplanted eyes there are small 
ones and large ones, and some with bits of brain tissue attached. Such 
results are most readily explained if we assumed that those cells which 
go to form the eye are already determined and that the line of separation 
between brain and eye cells is sharp and can be indicated by a line, as 
cd (Fig. 3°). Cuts separating the eye from the brain lateral to ed would 
leave varying numbers of optic vesicle cells attached to the brain and so 
give rise to regenerated eyes of various sizes, cuts along the plane cd 
would leave no cells for regeneration, while cuts median to the plane cd 
would include, with the transplanted eye, various amounts of brain 
tissue. As all these conditions are found, in the embryos experimented 
upon, it is but natural to conclude that the eye cells are already pre- 
determined, although on microscopical examination no line of demar- 
cation can be seen between brain and eye cells at the time of closure 
of the neural folds. This question will be more fully discussed in 
another paper on the origin and differentiation of the optic vesicle. 
The lateral surface of the optic vesicle at this stage is in direct con- 
tact with the inner layer of the ectoderm over a considerable area (see 
Fig. 3). There is very little mesenchyme at this side of the brain, but 
ventral to the brain in this region there is a considerable layer of mesen- 
chyme. ‘There are no indications of any changes in the ectoderm 
