416 Charles R. Stockard. 
may arise and differentiate independently of the optic vesicle 
stimulus. In these experiments it must be noted that Spemann 
operated with glass needles to remove the optic vesicle regions 
from the open medullary plate. Such an operation does not 
injure the lens-forming region of the ectoderm as Lewis’ experi- 
ment does and it is on this account, the writer believes, that the 
free lenses arise. 
The tendency of the ectoderm to form a lens independently 
is so delicately adjusted that a very slight injury or disturbance 
may suppress it, yet the same ectoderm may still have the power 
to form a lens in response to the stronger stimulus of the optic 
vesicle. So in experiments where the ectoderm has been cut or 
injured it loses the power to form free lenses even though the optic 
vesicle can stimulate a lens to arise from it. When the ectoderm 
is uninjured, as in some of King’s specimens, Spemann’s (07), 
and the writer’s, then free lenses do occur. 
We may imagine the lens to represent an ectodermal organ 
formerly of independent importance. However, it has now be- 
come so closely associated with the nervous portions of the eye 
that it arises whenever such a part meets the ectoderm, yet the 
lens retains to a feeble degree its impulse to arise independently 
of other eye parts. When it has once arisen it is perfectly capable 
of differentiation. Future experiments of removing the optic 
vesicle without injuring the ectoderm will probably demonstrate 
further this tendency of the lens to arise independently, just as 
Mencl’s observations and the writer’s experiments show for the 
fish, and many of the experiments mentioned show for amphibians. 
King (05) states that she had begun her series of experiments in 
1900. She destroyed the optic vesicles from the forebrain region 
in the closing meduilary tube with hot needles. Many of the 
embryos died as a result of theoperation. Theconclusionsreached 
by King are entirely opposed to those of Lewis and Spemann’s 
earlier results regarding the dependent origin of the lens. King 
found that in some of the embryos a lens-like body arose from the 
ectoderm on the side with an injured or unregenerated optic cup 
which did not come in contact with either the ectoderm or the 
lens-like body. Some of the specimens show a very suggestive 
