Warren Harmon Lewis 491 
increase in number they project more and more into the optic-cup cavity 
and form the lens-bud. It is possible, owing to the adhesion of the 
lens-plate and lens-bud to the retinal layer of the actively invaginating 
optic-cup that the latter exerts a pull on the lens-plate and helps in 
the evagination of the lens-bud. This same pull may also be the stimu- 
lus which causes the cells to multiply as they are elongated. These are, 
of course, very difficult points to prove. 
The first and most important early influence then which the optic 
vesicle exerts on the inner layer of the ectoderm is a stimulus causing the 
cells over the area of contact, and especially over the center of this area, 
to multiply faster than those in the region about. This is clearly indi- 
cated by several facts. There is at first no apparent alteration in 
the constitution of lens-plate or lens-bud cells. The cells of some of 
the small abortive lens-buds even several days after their formation 
are similar in staining reactions to the cells of the inner layer of the 
ectoderm. There is a difference in shape, but this is probably due to 
mechanical relations. Again the cells of the abortive lens-buds or 
vesicles are similar to the lens-buds or vesicles arising from mechanical 
injuries of the ectoderm. The fact also that these early lens-buds are 
not self-differentiating, points to the cells not being essentially different 
in structure from the cells of the inner layer. 
That the prolonged influence of the optic-cup does alter the structure 
and chemical constitution of the lens-cells would seem self-evident. 
CONCLUSIONS. 
The lens will not arise from the normal lens-forming region of the 
ectoderm without the contact stimulus of the optic vesicle on the inner 
layer of the ectoderm. The lens is not a self-originating structure. 
The lens will not develop, grow and differentiate, without the con- 
tinued influence of the optic vesicle and optic-cup. 
The lens is not a self-differentiating structure. 
Probably only the retinal portion of the optic vesicle is capable of 
stimulating lens-formation from the ectoderm. 
The size of the early lens-structure is due in part to the area of 
contact or adhesion between optic vesicle and ectoderm, and in part to 
the length of time the optic vesicle or optic-cup remains in contact 
by its retinal layer with the growing lens-structure. 
The intitial stimulus of the optic vesicle on the skin is such as to 
cause increase in the rate of cell-division at the place of contact, and 
may be only mechanical. 
