268 The Origin and Differentiation of the Optic Vesicle 
from the same optic vesicle lying in the same embryo just caudal to the 
regenerated one may be from two-thirds to seven-eighths the diameter of 
the normal one. Thus the regenerated and transplanted eyes may to- 
gether be nearly twice as large as an eye the same optic vesicle tissue 
would have produced tinder normal conditions, and the cutting away a 
portion of it must have disturbed in some way the regulative mechanism. 
We have already noted that in many of the experiments there is no 
regeneration of the eye and in others only regeneration of the optic 
stalk and in the remainder regenerated eyes of various sizes from ex- 
tremely small ones to some nearly as large as normal. Amount of brain 
tissue cut away was never: very extensive, and in some of the experiments 
there is no evidence of any excision of brain tissue, and this applies to 
all grades of the regenerated eyes, even in cases where there was no 
regeneration, and in no instance so far as I am aware was there any brain 
tissue taken from the opposite half of the brain. The age of the embryo 
within the limits of the time they were allowed to live after the operation, 
2 to 20 days, does not influence these results. 
Those experiments in which there was no regeneration or only regen- 
eration of the optic stalk seem to me to indicate very clearly that the brain 
has no power of regenerating an eye. Bell (experimental studies on the 
development of the eye and nasal cavities in frog embryos,” preliminary 
communication) makes the statement that “the retina may certainly be 
regenerated in very young embryos after removal of its entire anlage.” 
He removed, after reflecting a skin flap, the anterior lateral one-half of 
the brain and optic vesicle of embryos of rana esculenta. These embryos 
were from 2.5 mm. to 3.5 mm. in length, the tail bud had developed, the 
otic vesicle is present, and the lens is to be recognized as a slight thicken- 
ing of the ectoderm,” if so he used a slightly older stage than I have 
used. At Bell’s stage when the lens-bud is present the retinal portion 
of the optic vesicle is firmly adherent to it, in rana palustris, rana syl- 
vatica, and amblystoma, and this, I judge, must be the condition in rana 
esculenta also. Jn rana palustris, for example, it is very difficult and 
often impossible to reflect a skin flap from over the optic vesicle and not 
leave some of its cells attached to the skin flap, and I imagine in Bell’s 
experiments some eye cells were left attached to the skin flap which was 
then replaced over the brain and pressed down. These cells thus devel- 
12” Anat. Anz., XXIX, 1906, p. 186. 
* See his article in Arch. f. Mikr. Anat. u. Entwickelungsgeschichte, Bd. 68, 
1906, pp. 279-280, Figs. 1 and 2. 
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