302 Experimental Zoology 
it has been found that after the development and differentiation 
of a part have begun it continues to self-differentiate even if re- 
moved to another part of the embryo. As an example of corre- 
lated development, an experiment carried out by Lewis may be 
cited. 
Lewis has shown that any part of the ectoderm of the embryo 
of the frog can produce the lens of the eye, provided a piece of 
the optic vesicle is transplanted beneath it. The result shows 
that the optic vesicle exerts a formative stimulus on the ecto- 
derm, calling forth the lens formation. ‘This case is the best- 
ascertained example of the so-called formative stimulus and 
one of the few indisputable cases of the sort. Although em- 
bryologists have some reason to conclude that similar stimuli 
may play an important réle in development, it has been found 
difficult to obtain evidence of this kind of action. On the other 
hand, the power of independent self-differentiation of the parts 
has been demonstrated in a large number of cases. Both 
principles appear to play a réle in the development of the 
embryo. 
In a somewhat different way Lewis has examined the same 
problem. He removed the ectoderm lying over the eye vesicle 
and transplanted there a piece from another animal — a differ- 
ent species, in fact, whose skin was differently colored. The 
lens developed at the proper time from the grafted piece. 
In contrast to these results Lewis found that pieces of the eye 
vesicle transplanted in different parts of the body underwent 
there self-development and differentiation. 
Harrison has studied the perplexing problem of the normal 
growth of the nerves in the embryo, by uniting parts of the bod- 
ies of young tadpoles in such a way that the nerve must extend 
into a territory that is normally foreign to that nerve. Since 
the nerve extends into the new region, Harrison concluded that 
its growth takes place by its own substance extending outward, 
and not by the addition of cells in the new part. 
Braus has recently studied the same problem in another way. 
The limb bud of the tadpole, at a very early stage in its develop- 
