150 Lens-Formation from Strange Ectoderm 
vader constantly struggling to penetrate between and within the various 
organs of the body, and so pushing them apart. in one way or another. 
In diseased conditions, as of the liver or kidney, the resistance against 
its invasion is lowered, the balance lost, and as a result a connective 
tissue invasion occurs. In the gradual separation of the otic vesicle 
from the ectoderm we have an example of this action of the mesenchyme. 
The growth of the otic vesicle is perhaps a factor in some cases, for at 
the time of transplantation it consists of merely a thickening of the 
ectoderm (see Fig. 1), its great increase in size, as in Figs. 44, 45, and 
47, may have helped some‘in forcing the transplanted eyes away from 
the ectoderm. 
The fact that in the younger embryos the eyes are in contact or close 
to the ectoderm and that in the older embryos there is a progressive in- 
crease in the thickness of the mesenchyme would seem to speak in favor 
of a contact between ectoderm and optic vesicle at an early stage. Again, 
there are in this series a number of experiments in which the attempt 
was made to transplant the optic vesicle some distance beneath the ecto- 
derm and in most of these the eye apparently remained deeply seated and 
without a lens, the eye not being able to stimulate at a distance lens- 
formation from the ectoderm. 
There is, also, no indication in these early stages of lenses arising from 
the edge of the optic cup and the fact that the lens is often nearly as large, 
or as large, as normal and as far advanced or nearly so would seem to 
exclude this mode of origin, especially as in the early stages of the 
transplanted optic vesicle there is no distinct papillary margin from 
which the lens in older amphibian embryos has been found arise. 
During the operation of transplanting the eye, the ectoderm in the 
region of the transplanted eye, or at the edge of the pocket, was often 
injured in such a way as to often bring pieces of ectoderm into the 
region of the transplanted eye. Such pieces of ectoderm vary much in 
size and shape, they may or may not remain attached to the ectoderm. 
If the optic vesicle comes into contact with such pieces of ectoderm a 
lens may arise from that portion of it in contact with the retinal layer. 
Such long ectodermal processes may remain attached in the ectoderm and 
a lens form from only a portion of it, as in Figs. 49, 50, and 51. The 
lens in Fig. 51 is connected on one side with a long process to the ecto- 
derm and on the other with two irregular processes which dangle in the 
cavity of the optic cup. The lens evidently only formed from a small 
part of this process. 
The lens shown in Fig. 45 may have arisen from the ectodermal 
