256 _ Origin and Differentiation of the Lens 
lens-fibers. The medial pole is quite obliterated by the overgrowth of 
the layer of cuboidal epithelium that surrounds the lens completely. 
In the oldest stage (XIV) operated upon, the lens was in very slight 
contact with the overlying ectoderm (Figs. 35-36). 
The lenses of an embryo (Experiment XIV,,) allowed to live five 
days show no difference in size, both measuring 150 », but the right one 
has a complete covering of epithelium around the nuclear mass in the 
center (Fig. 37), and is not widely separated from the ectoderm. A 
small bit of the eye that had been left in contact with the lens, appears 
te have had little or no influence upon it. 
The right lenses of embryos (Experiments XIV, ,) killed eight and 
ten days after the operation are smaller than the normal, one is 20, and 
the other 30, less in diameter. Both right lenses (Figs. 38-40) are 
surrounded with complete layers of columnar epithelium. However, in 
the eight-day embryo there still remains an indication of the medial 
pole, while in the older embryo the pole is quite obliterated. The lenses 
are not widely separated from the overlying ectoderm, are smaller and 
in marked contrast to the perfect ones (Fig. 39). The fibers within the 
eight-day lens are fairly normal, but are beginning to show some degen- 
eration, while those in the 10-day lens show considerably more. 
An embryo (Experiment XIV,,) of this stage (XIV) that was al- 
lowed to live 30 days affords a very striking instance of the lack of de- 
velopment of the lens when the influencing optic cup is remoyed. ‘The 
normal lens measures about 220, in diameter (compare Fig. 9), and 
is only 20m larger than the right lens. However, the latter consists 
only of a vacuolated mass of degenerated lens-fibers surrounded by a thin 
layer of epithelium (Fig. 41). The lens has separated completely from 
the ectoderm, but is still adjacent to same. It has grown considerably, 
but in order to accomplish perfect differentiation, the influencing me- 
dium of the optic cup was wanting, and the result was the extensive 
degeneration of the lens fibers with the complete obliteration of the 
medial pole. 
Whether this continued influence of the optic cup upon the develop- 
ing lens is a specific one or not, can be determined perhaps only by fur- 
ther experimental work. It may be that the nasal-pit, otic vesicle, or 
brain can exert such an influence, as to cause the lens to develop and 
differentiate normally. Lens-plates, lens-buds, Jens-vesicles, and well 
differentiated lenses with some of the surrounding ectoderm might be 
transplanted into the brain itself, and thus determine whether there 
also the lens can find the influences essential to its normal differentia- 
