264 The Origin and Differentiation of the Optic Vesicle 
remain as a vesicle, connected with the ventricle of the brain by a wide 
opening as seen in Fig. 9. Here the various layers of the retina have 
developed, but the rods and cones and pigment layer are not in contact. 
The pigment layer does not possess, as in a normal eye, the numerous 
fine processes which run in between the rods and cones (compare Figs. 
11 and 12). In Fig. 9 the section is directly through what would have 
been the cleft region, for we see here on the ventral side, the retina con- 
tinuing without the intermission of the pigment layer directly into the 
optic stalk and here thesoptic nerve leaves the retina and enters the optic 
stalk. A neighboring section shows the fovea centralis, and still more 
lateral sections at one side or other of the cleft region show as in Fig. 
10 a considerable extent of pigment layer on either side of the retina. 
This eye illustrates again the idea that there is not much of any corre- 
lation within the eye which determines the fate of the various cells of 
the optic vesicle, as for example, the outer pigment layer is not de- 
pendent for its differentiation on a contact relationship between it and 
the retina. + 
The choroidal fissure is present in a number of the transplanted eyes, 
both regular and irregular ones. In Fig. 17 it is cut at an angle and 
cannot be clearly shown in a section, but in studying the series its pres- 
ence is evident. Fig. 16 is from a section directly through the fissure of 
this rather irregular eye. Continuous with this region and extending to 
the pigment layer is an area of what is evidently optic stalk tissue. 
An optic stalk is plainly to be seen in a number of the eyes (see Figs. 
13, 17, 18, 19, 20, 21, and 22). A piece of brain tissue may or may 
not be attached at the other end of the optic stalk. There is no brain 
tissue connected with the eye or optic stalk of Fig. 17 for example. On 
the other hand, the optic stalk may develop without an eye, as in Fig. 
24. In this experiment the optic vesicle was cut away in such a manner 
as to leave no tissue for the regeneration of an eye and yet there is con- 
nected with the brain in the normal position a short, perfectly normal 
optic stalk. Fig. 25 shows another very similar example. The cut 
separating the eye from the brain in these experiments was probably in 
such a position as to leave no cells behind for regeneration of the eye; 
a few cells, however, were left attached to the brain which were destined 
to form optic stalk and have done so in the absence of the eye. In 
many of the transplanted eyes the optic stalk differentiates (see Figs. 
13, 17, 18, 19, 20, 21, and 22) and becomes sharply distinguishable from 
the pigment layer to which it is usually attached. The presence or absence 
