Warren Harmon Lewis 479 
were killed 3 days after the operation, and on the normal side in each 
is a well-formed lens (Fig. 59). The regenerating eyes in these embryos 
are in contact with the ectoderm by the outer layer, which does not 
seem to possess the power of stimulating lens-formation. 
In 34 embryos of rana palustris killed at varying ages after partial 
extirpation of the optic vesicle there are regenerating eyes of various 
sizes without lenses or traces of lens-formation. 
Regenerating eyes in contact with the ectoderm for a sufficient length 
of time can stimulate lens-formation, but in the above experiments the 
absence of lens-formation is to be explained through want of contact 
between eye and ectoderm, or to the contact not having been of sufficient 
duration; or to the contact having been over too small an area, or to 
contact by the outer layer of the optic vesicle, which does not seem 
to possess the power of stimulating lens-formation. The adhesion which 
ordinarily takes place between optic vesicle and ectoderm before lens- 
formation is probably an important factor, and if this is interfered with 
even though contact may exist, it is possible that a lens would not arise. 
These experiments indicate that the lens is not self-originating, and 
that the regenerating eye cannot stimulate lens-formation when separated 
from the ectoderm by mesenchyme. 
Lens-formation Associated with Regenerating Eyes. 
In 25 embryos of rana sylvatica killed from 3 to 16 days after partial 
extirpation of the optic vesicle there are regenerating eyes of various 
sizes associated with lenses or lens-like structures of various stages‘ and 
sizes (Figs. 17, 27, 30, 34, 35, 36, 37, 64, and 65). 
Small and imperfect Jens structures are often associated with some 
of these smaller regenerated eyes. The larger regenerated eyes, how- 
ever, give rise to normal lenses, indicating thereby that the operation 
itself, unless a considerable portion of the optic vesicle is cut away, does 
not interfere with lens-formation from the skin flap. These large re- 
“It has seemed convenient to divide the development of the lens into 
several stages: (1) the lens-plate, or the thickening of the inner layer of the 
ectoderm; (2) the lens-bud (Fig. 67), projection of this thickened area until 
its separation from the ectoderm; (3) the lens-vesicle (Figs. 43, 59, 66, 68, and 
69), the vesicle-like structure from the time of its separation from the ecto- 
derm until the differentiation of the anterior epithelial layer and lens-fibers; 
and (4) the lens (proper), the earlier stages of which are seen in Figs. 61 
and 70, and the later stages in Figs. 38, 74, 75, 76, 77, and 78. 
