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 Levels 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, calhng 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 role 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 rdle 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 territor\^ 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- 



