486 The Origin and Differentiation of the Lens 
some embryos an irregularity in the skin flap may have prevented its 
contact with the stump of the optic vesicle. 
From a study of the abortive lenses and lenses of various sizes with 
regenerating eyes it becomes apparent that not only is contact between 
eye and ectoderm necessary for the initial origin of the lens, but that 
the size of the lens-plate and lens-bud is dependent upon the area of 
contact, also the development of the lens is dependent on the continued 
influence of the eye. The fact that a regenerating eye is often separated 
from the ectoderm by mesenchyme is no indication that it was never in 
contact with it. Mesenchyme always grows in between the normal eye 
and ectoderm (Figs. 77 and 78), yet one would not hesitate to state that 
at the time of lens-formation the normal eye was in contact with the 
ectoderm. The operations do not interfere very much with the growth 
of mesenchyme in the region of the regenerating eye, and as the regen- 
erating eyes are smaller than normal and are attached to the brain, 
we should expect to find them more often separated from the ectoderm 
by mesenchyme than the normal ones, and also at a greater distance from 
the ectoderm than in normal eyes. This ingrowth of mesenchyme 
often interferes with lens-formation, if the ingrowth takes place before 
the lens has started the latter will fail to appear, but if the ingrowth 
of mesenchyme occurs after the lens has begun to form, various degrees 
of abortive lens-formation occurs, these depending, in part, on the 
stage of development of the lens-bud at the time of the ingrowth of 
the mesenchyme, separating the lens from the eye. In some instances 
the lens-bud may be pulled out into a long process, owing to the adhesion 
between eye and lens-bud. As the eye is attached to the brain the pres- 
sure of the growing mesenchyme would tend to force the ectoderm away 
from it and thus either stretch out the lens-bud or separate the eye 
from the lens-bud or the lens-bud from the ectoderm. Figs. 31 and 33 
are from experiments where I believe the optic vesicle was originally 
in contact with the ectoderm, stimulated a small lens-plate and small 
lens-bud, but with the growth of mesenchyme the small eye has been 
pushed some distance from the ectoderm, and owing to the adhesion 
between the lens-bud and eye the former was pulled out into an elongated 
form. Following this has come more or less separation of the lens-bud 
and optic-cup, and owing to the original small size of the lens-bud 
and its later separation or partial separation from the optic-cup' re- 
tardation in development has occurred. The embryos (DF,, and DF,,) 
from which Figs. 31 and 33 are taken were killed 4 days after the 
operation and are in contrast to the conditions found in another embryo ~ 
