260 The Origin and Differentiation 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 walls of the 
optic vesicle are of about the same thickness as the adjoining ventral 
wall of the brain. There is at this time no indication of any histological 
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 the 
following experiments. A skin flap was turned back from over the optic 
vesicle without injury to it, and the eye was then cut off close to the 
brain and transplanted in most of the experiments beneath the ectoderm 
in various regions of the same or another embryo of the same age or 
even an older embryo. This is the same operation I have referred to in 
my papers on the origin of the lens.” It is impossible in working with 
such small objects to do each operation exactly alike, nor did I attempt 
especially to make the cut separating the optic vesicle from the brain in 
the same place each time. There were also slight variations in the ages 
of the embryos and in the extent of evagination of the optic vesicle. 
These optic vesicles transplanted into various regions of the same or 
other embryos continue to differentiate in the new environment, some- 
times at about the same rate as if they had remained in the normal posi- 
tion and attached to the brain. Sometimes, however, there was consid- 
erable retardation in the rate of differentiation of the various layers of 
the retina during the early stages, but in the older eyes no distinction 
can be noticed between the amount of differentiation of transplanted 
and normal eyes. Such transplanted eyes, whether in the otic region, 
the abdominal or tail regions of the same embryo or an older embryo of 
the same or another species continue to increase in size, to invaginate 
whether a lens is formed or not, and the pigment layer and various layers 
of the retina develop in a perfectly normal manner, provided the trans- 
planted eye was not severely injured or distorted at the time of the 
operation. I have already figured a number of such transplanted eyes.’ 
It often happened that such transplanted eyes were distorted and torn 
during the operation and as a result many of them are more or less 
irregular in shape, especially as regards the cup cavity. But even in 
these distorted ones the various layers of the retina are present though 
2Am. Jour. of Anat., Vol. III, p. 507, 1904; Vol. VI, p. 4738, 1907; Vol. VII, 
p. 146, 1907. 
Lewis, Am. Jour. of Anat., Vol. III, 1904, Figs. 20 and 22, p. 518, 24 and 25, 
p. 519, and 28, p. 521; ibid., Vol. VII, 1907, pp. 159-169, Figs. 33, 34, 35, 37, 38, 
40, 41, 42, 44, 47, 50, 52, 57, 61, 68, but especially Figs. 39, 42, and 44, p. 161. 
