490 The Origin and Differentiation of the Lens 
explanation of contact without lens-formation, provided lens-forma- 
tion is dependent upon adhesion. The size of the lens-plate and 
lens-bud, that is the number of cells of the inner layer of the ecto- 
derm which take part in their formation, is dependent then, upon the 
area of contact of the retinal portion of the eye and probably upon the 
area of adhesion between the retinal portion of the eye and the ectoderm. 
The size of the lens-bud and lens vesicle is dependent also upon 
the increase in the number of cells, and this is dependent upon the 
continued influence of the optic-cup. For how long a period contact 
is necessary or for how long a time the lens must maintain its normal 
relations with the optic-cup in order that it may develop and grow in- 
dependently I am unable to answer at present. Whether a very small 
lens-bud would ever form a normal sized lens, even if its relations with 
the small optic-cup were maintained as in a normal eye for a long period, 
I am also unable at present to determine from my specimens. 
Still other factors may play a role in regulating the size of these lenses 
as retarded stimulation of the ectoderm by the regenerating eye. It is 
possible, of course, that as the embryo gets older the ectoderm responds 
less and less readily to the stimulation of the optic vesicle, or that older 
optic vesicles stimulate less readily the ectoderm to form lenses. It is 
not possible, however, to determine this from my experiments. 
The Nature of the Initial Stimulus of the Optic Vesicle Leading to 
Lens-formation. 
Under normal conditions that portion of the optic vesicle which forms 
the retina first comes into contact with the inner layer of the ectoderm, 
the two soon become adherent and they can only be separated with 
difficulty. Then follows a thickening of this inner layer, which is 
greatest nearer the center of this area of contact. Accompanying this 
thickening and probably in part, at least, the cause of it, is an increase 
in the number of cells in this area. Jn order at first to accommodate 
the increase in the number of cells they, through mutual lateral pressure 
on each other, are compressed in the axis parallel to the ectoderm and 
elongated in the perpendicular axis. ‘This gives the formation of the 
lens-plate. Accompanying this thickening there takes place the in- 
vagination of the optic vesicle. The invagination of the optic vesicle 
into the form of the optic-cup is an active process on the part of the 
optic vesicle, as I have already pointed out.” As the cells of the lens-plate 
1 Am. Jour. of Anat., Vol. III, Proc. of the Ass. of Am. Anat., p. XIII. 
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