Warren Harmon Lewis : 485 
self-differentiation.” Again among my experiments on rana palustris 
and rana sylvatica there are numerous instances in which the lens-bud 
has apparently ceased to develop, owing, I believe, in part, at least, to 
a shifting away from the bud of the small regenerated eye, or irregular 
transplanted eye, and to an ingrowth of more or less mesenchyme — 
between the two; in others irregular changes in the form of the eye which 
shifts the contact of the lens from the retinal to the outer layer leads 
to similar abortion in lens growth (see Figs. 58, 57, 18, 20, 25, 27, 30, 
34, and 40. 
Another possible explanation of King’s “ lens-like structures” is that 
they are injury buds due to injury of the ectoderm during the operation, 
and are so, perhaps, similar in origin to some of those I have already 
called attention to in a preceding section. 
My experiments on the extirpation, partial or entire, of the optic 
vesicle, were performed on embryos a few hours older than those used by 
King. The possibility or probability that the lens-forming ectodermal 
cells possess at the stage King used any unusual powers of self-differ- 
entiation into a lens is scarcely worth serious consideration, as one 
would naturally expect this power of self-origination to show itself at 
the stage I used more readily than at the stage King used. Spemann’s 
experiments were on embryos younger than those used by King, yet 
he had no indications of self-origination of the lens. In view of these 
various facts of abortive lens-formation it seems to me much better 
to explain King’s lens-like structures in one of these ways rather than 
concluding that the lens is self-originating. 
Can the Optic Vesicle Stimulate Lens-formation from a Distance or is 
Direct Contact of the Optic Vesicle with the Ectoderm Necessary? 
We have noted that in 85 embryos, 51 in rana sylvatica and 34 in 
rana palustris, there were regenerating eyes of various sizes without 
lens-formation. Such eyes are usually separated by mesenchyme from the 
ectoderm, yet eyes may come into contact with the ectoderm by the 
outer layer without stimulating lens-formation. ‘The regenerating eyes 
without lenses are usually smaller than those with lenses. Owing to 
the small size the mesenchyme is much more likely to grow in between 
the eye and ectoderm and then to prevent contact between the two. Tiss 
°Proc. Ass. of Am. Anatomists, December, 1906. Jour. of Anat., Vol. V, 
p. XI; also Am. Jour. of Anat., Vol. VI, No. 2. 
