Independent Development of the Lens. 395 
stimulus of the optic vesicle by forming a lens. This point is 
important, for herein the writer believes, after a study of his 
own experiments and the results of other workers, lies the explana- 
tion of many discrepancies in the operation experiments on the 
lens. When the operation is so performed that the ectoderm 
must be folded away in order to extirpate the optic vesicle and is 
then returned to its place, free lenses have failed to occur, although 
an optic vesicle may still have been able to derive a lens from 
this replaced ectoderm. On the other hand, when the early open 
medullary plate is operated on so as to remove the optic vesicle 
areas, the ectoderm of the head wall is sometimes left uninjured 
and from it may arise free lenses. The free lenses of King’s 
experiments arose in embryos which were operated on dorsally 
to burn out the optic vesicle areas of the partially open medullary 
tube. The lateral head ectoderm was probably uninjured in 
some of the specimens. In Spemann’s more recent experiments 
the early open medullary plate was operated upon directly to 
remove the optic vesicle areas; in such experiments free lenses 
arose from the uninjured ectoderm. Experiments on other spe- 
cies at such stages and in a similar manner will probably give like 
results. 
In the experiment of removing the optic cup and leaving a par- 
tially differentiatied lens, this lens may have degenerated or 
ceased to differentiate on account of the injury it suffered by the 
operation, the absence of the optic cup not affecting it. It is 
unquestionably true that in Some amphibians and fishes the lens 
is capable of perfect self-differentiation. 
The optic cup does not exert complete control over either the 
size or shape of the optic lens. Numerous points of detail are 
also elucidated by the study of the optic organs in artificially 
produced fish monsters which are either blind or present various 
eye defects. 
The experimental part of this investigation was conducted dur- 
ing the summer of 1909 in the Marine Bioligical Laboratory at 
Woods Hole, Mass., while occupying one of the rooms of the 
Wistar Institute. 
