478 The Origin and Differentiation of the Lens 
another embryo killed 5 days after complete extirpation of the optic 
vesicle. A portion of the optic stalk has regenerated, but there is no 
trace of a lens in this region. 
This resulting absence of lens-formation after complete extirpation of 
the optic vesicle in rana sylvatica is in entire accord with my previous 
work on rana palustris." Some new experiments on rana palustris give 
me now 21 examples of complete extirpation of the optic vesicle by 
the above operation, in which there was failure of lens-formation, asso- 
ciated with complete absence of the eye. 
Similar operations on amblystoma punctatum by Le Cron’ likewise 
demonstrate the lack of lens-formation after complete extirpation of the 
optic vesicle at this early stage. 
Spemann’s experiments” on rana fusca, made on embryos younger 
than those used by me, also show that if the eye spot is killed on the 
open medullar plate, the lens fails to appear. In none of my experi- 
ments are there to be found lenses or lens-like structures in the normal 
lens region when the optic vesicle fails to regenerate. 
As the lens often does arise when the eye regenerates its absence is 
not due to the operation itself but to the lack of influence of the 
extirpated optic vesicle. These experiments indicate very clearly that 
the lens is not a self-originating structure. 
Absence of Lens-formation After Partial Extirpation of the Optic Vesicle. 
In 51 of the embryos of rana sylvatica there was more or less regen- 
eration of the eye without any indications, however, of lenses or be- 
ginning lens-formation. Figs. 7, 8, 9, and 10 are from sections through 
such regenerating eyes of embryos killed from 3 to 5 days after the 
operation. They are among the larger of the regenerating eyes without 
lenses, but are much smaller, however, than normal eyes of the same 
age (see Figs. 8, 75, and 77). These regenerating eyes are separated 
from the ectoderm by mesenchyme and were probably never in contact or 
not in contact with the ectoderm for a sufficient length of time to stimu- 
late lens-formation. The majority of regenerating eyes without lenses 
are separated from the ectoderm by mesenchyme; a few exceptional 
ones, however, were found to be in contact with the ectoderm (Figs. 
11, 12, and 13). The embryos from which these figures were taken 
* See figure 6, p. 510, and figure 9, p. 512, Am. Jour. of Anat., Vol. III, 1904. 
5Am. Jour. of Anat., Vol. VI, 1907. 
®°Verhandl. der Anat. Gesell., 1901. 
