Wilbur L. Le Cron 253 
the overlying ectoderm, and is of good size, but is not so far advanced 
ir point of differentiation as the normal left lens. A well defined me- 
dial pole is to be seen, but this appears to have stopped development 
when compared with the rapid advancement of the pole of the opposite 
lens to form fibers. Had the embryo lived longer the lenses would have 
shown a much greater difference, due, as in other instances, to the absence 
of the influencing optic cup. In the nine- and eleven-day experiments 
the right lenses have separated from the ectoderm. They have epithelial 
coverings one layer in thickness, and in the centers are masses of lens- 
fibers more or less degenerated. The normal lenses on the left sides 
measure about 170, while the right lenses are much smaller, being 
only 120 » in diameter. 
In such instances, where only a slight regeneration of the eye occurs, 
the lenses may at first be rather normal in appearance, but later they 
invariably show degeneration, which is due, I believe, to the removal of 
the optic cup. The partial influence of the bit of the optic vesicle is 
not sufficient to further lens development. The experiments in 
which only partial regeneration occurred are likewise in accordance with 
the conclusion drawn, that the optic vesicle is necessary for the subse- 
quent differentiation of the developing lens. 
In a still older stage (XII) the gill mass shows three divisions, and 
the tentacles have begun to develop. The lens-vesicle is quite prominent. 
It has pinched off from the inner layer of the ectoderm, but is still 
tightly pressed against the same (Figs. 20-21). Twelve experiments 
were made upon embryos of this age, and five of these show some regen- 
eration of the eye. 
An embryo (Experiment XII,,) of this series was killed two hours 
after the operation. Within this time the skin had healed at the place 
of incision. No special change had taken place in the’ right lens, and 
it appears perfectly normal. Both lenses are still in contact with the 
overlying ectoderm, and show well defined medial poles with the begin- 
ning of lens-fiber formation (Figs. 22-23). 
In another embryo (Experiment XII,,) which was allowed to live 
two days, the lenses have just separated from the ectoderm, and show 
no special difference in development (Figs. 24-25), although from the 
appearance of the right lens the loss of influence of the optic cup is just 
beginning to be felt. Its medial pole is very definite, but the shape and 
arrangement of the nuclei are somewhat abnormal. 
Instances like the above indicate further that the operation of turning 
the skin flap forward does not interfere with the developing lens, which 
always has a certain amount of independent self-differentiation even 
