488 The Origin and Differentiation of the Lens 
that action at a distance is a process not very likely to occur. ‘The larger 
the regenerated eyes the greater the number that show lenses. This 
is what one would expect if contact were necessary, for the larger the 
regenerated eye the greater its chance for prolonged contact with the 
ectoderm. 
We are practically forced to conclude that actual contact between optic 
vesicle and ectoderm is necessary for lens-formation, as this seems to 
be the common factor lacking in the above 85 examples of regeneration 
of the eye without lens-formation. 
The lens is then dependent on direct contact of the retinal portion 
of the optic-cup or vesicle on the inner layer of the ectoderm for its 
origin. Abnormal relation between the early lens-plate or lense-bud 
and eye is accompanied by more or less abnormal or abortive lens- 
formation. The size of the lens, as will be shown further on, and even 
the differentiation of the lens, are dependent on the continuance of the 
normal relations between the optic-cup and developing lens. 
The Lens is Not Self-differentiating. 
An optic vesicle transplanted into the region of the otic vesicle, for 
example, will continue its growth and differentiation independently of 
any especial environment. Invagination, differentiation of the various 
layers of the retina, and the formation of the optic nerve takes place 
as readily as when the eye has its normal position and attachment to 
the brain. It is a remarkable self-differentiating organ. The behavior 
of the lens is in marked contrast to eye. Le Cron has shown that in 
amblystoma removal of the optic-cup without injury to the developing 
lens is followed by abortive lens-formation. The earlier the stage at 
which the eye is removed the less power of growth and differentiation 
the lens rudiment possesses. Even when the optic cup is removed after 
the lens vesicle has separated from the ectoderm its progressive growth 
and differentiation soon cease and ultimately degenerative changes occur 
in the lens-fibers. 
The numerous instances of abortive lens-formation in rana are con- 
clusive evidence that here too the lens is not a self-differentiating struc- 
ture, but is dependent, not only on the presence of the optic-cup, but 
the maintenence of more or less normal relations between the two for 
a considerable period of time. Disturbance of such relationship by 
irregular invagination or lack of invagination of the optic vesicle and 
by ingrowth of mesenchyme results, as in complete extirpation of the 
optic cup, in abortive lens-formation. 
