474 The Origin and Differentiation of the Lens 
of time in order that a perfect lens may form. My experiments throw 
some light on the nature of the earlest influence of the optic vesicle 
on the ectoderm; lens-like structures of the ectoderm can be produced 
by mechanical injuries of the ectoderm, as with a needle or other 
instrument. ‘These lens-lke structures consist merely of a proliferation 
of cells of the inner layer of the ectoderm into small buds, small solid 
bodies or small vesicles, but they do not show signs of differentiation 
into lens fibers, ete. Their great similarity to some of the earlier 
stages of abortive lens-formation suggests the idea that the initial stimulus 
of the optic vesicle is such as ‘to cause at first only an increase in the 
rate of cell division in the area of contact of the ectoderm, and it may 
be that the earhest stimulus of the optic vesicle is purely mechanical. 
It was shown in my previous paper that probably any portion of the 
inner layer of the ectoderm is capable of giving rise to a lens when 
properly stimulated. There is, then, no especial predetermined group of 
cells which must be stimulated, in order that a lens may arise. There 
cannot be then in the ovum or fertilized egg, substances, either proto- 
plasmic or chromatic, or otherwise, which represent the lens in the 
sense in which Conklin has found certain definite kinds of protoplasm 
that differentiate into the central nervous system, the muscular system, 
ete. Such tissues or organs as the central nervous system, the muscular 
system, the ectoderm, etc., which are in a way represented by substances 
in the egg, possessing more or less power of self-differentiation, might 
be designated as fundamental or primary tissues and the others as the 
lens and cornea as secondary tissues. The latter would be dependent 
for their origin on reactions between the primary tissues during the 
course of development, or if the reaction takes place very early, between 
substances which represent them in the ovum. How large this group 
of secondary tissues is can only be determined by experimentation. That 
the cornea belongs to this class has been clearly shown by my experi- 
ments.” 
The present paper is concerned for the most part with the effects 
on lens-formation after total or partial extirpation of the optic vesicle 
with total absence or varying degrees of regeneration of these eyes. 
Anatomy of the Eye Region at the Operating Stage. 
In embryos of rana palustris and rana sylvatica at about the time 
of, or shortly after, the closure and beginning fusion of the neural folds, 
the optic vesicle projects from the sides of the brain and produces a 
° Jour. of Expt. Zo6l., Vol. II. 
