480 The Origin and Differentiation of the Lens 
generated eyes with lenses in rana sylvatica are very much the same as 
in rana palustris (see Figs. 67, 69, 73, 74, and 76). 
The sizes of the regenerating eyes during these early stages seem 
to be dependent much more upon the amount of eye tissue left attached to 
the brain, during the operation, than upon the length of time the embryo 
is allowed to live. 
In 76 embryos of rana palustris lenses or abortive lenses of various 
sizes and stages are associated with.regenerating eyes. In some of 
the smaller regenerating eyes the lens-bud or vesicle is very small and 
does not show much differentiation (see Figs. 18, 19, 20, 21, 23, 24, 25, 
26, 31, 32, 33, 40, 41, 60, 61, 62, 63, 64, and 65. In the larger regen- 
erated eyes the lens-buds, or vesicles, or lenses approach more the normal, 
as in Figs. 67, 69, 71, 72, 73, 74, and 76. These experiments indicate, 
as in rana sylvatica, that the failure of lens-formation, when the eye 
fails to regenerate or only regenerates a little, is not due to the mere 
reflecting of the skin flap and replacing the latter, but to the lack of 
the proper stimulus to the ectoderm. There is every reason to believe that 
in these experiments the regenerating eyes were in contact with the ecto- 
derm and then stimulated lenses to form, their sizes depending upon 
the area of contact. between optic vesicle and ectoderm and upon the 
duration of this contact. And the fact that the eye, after the lens 
has been stimulated to arise, is separated from the ectoderm by mesen- 
chyme is no indication that they were not at one time in contact. I 
see no other way of explaining why some regenerated eyes are without 
lenses and others have them, except in this manner, as the location of 
two eyes may be almost exactly the same at the time of killing the 
embryo, yet one may have a lens and the other not. 
Abortive Lenses with Regenerating Eyes. 
In both rana palustris and sylvatica there are a number of very 
curious small lens-buds associated with the small regenerating eyes 
(Figs. 18, 19, 20, 21, 23, 24, 25, 26, 27, and 30). Such abortive lens- 
buds are merely solid outgrowths of the inner layer of the ectoderm, and 
are very much smaller than the normal lenses; they show no especial 
indication of differentiation into lens-like structures, yet they were 
undoubtedly caused by the small regenerating eyes. Figs. 18, 20, 28, 
36, and 37 show only small and imperfect areas of contact between 
optic vesicle and ectoderm, and to this is due, in part, the small size 
and imperfect development of the lens-buds. The ingrowth of mesen- 
