418 Charles R. Stockard. 
that of the Florida burrowing lizéard, Rhineura. In the adults of 
Rhineura, Eigenmann finds lenses sometimes present and sometimes 
absent and when present they are very variable. ‘Somany examples 
are known of the traces of rudimentary organs in the embryo 
which are entirely lacking in the adult that the above cases of 
the lens are not at all strange and at present it seems that this 
is the most satisfactory explanation of their behavior. 
The more recent experiments of Spemann (’07) so frequently 
referred to, may be briefly described. Embryos of Rana escu- 
lenta were operated upon with glass needles so as to remove the 
optic vesicle areas from the open medullary fold (his former 
experiments were made with hot needles, which doubtless injured 
more of the surrounding tissues). 
After allowing these embryos to develop for several days and 
then studying them in section it was found that in one case on the 
side of the head lacking an optic vesicle a true lens-bud was still 
in connection with the ectoderm. In an older embryo a free lens 
vesicle was found, and finally, in a still older specimen a lens was 
buried in connective tissue on the eyeless side of the head yet 
it possessed fully formed lens-fibers. Thus, Spemann, five years 
after his first paper on the dependent origin of, the lens, now con- 
cludes that the lens in Rana esculenta is self-originating and 
self-differentiating. He also accepts Mencl’s case of the inde- 
pendent lens in Salmo salar. 
We have thus seen in a rather cursory survey how the lens 
problem has arisen and how experiments have built up first one 
side of the question and then the other, and many may also feel 
that the final word is yet to be added. Nevertheless, it is true 
that at this stage of the investigation it has been clearly demon- 
strated in several groups of animals that the ectoderm possesses 
the power independently to originate a lens which subsequently 
develops into the transparent refractive organ usually found 
within the mature eye. The manner of origin and differentiation 
of the lens may easily differ in different animal species, and the 
statement would not be warranted that all of these facts apply 
to animals in general. 
The lens is self-originating and self-differentiating yet it is more 
