INFLUENCE OF ENVIRONMENT 333 



taken place. Thus if a right hmb is trans- 

 planted to the left side of the body after it 

 has begun to differentiate it remains a right 

 limb and is not modified by its new relations 

 (Harrison) ; if the cleavage cells are already 

 differentiated in the four-celled stage, each 

 cell when separated from the others will give 

 rise to only one-quarter of an animal. In short 

 the organ or cell is already set, or fixed, or 

 differentiated to such an extent that it can not 

 change its fate even though its environment 

 should change. Such cases are known as "self 

 differentiation. ' ' 



Many students of the physiology of devel- 

 opment have been led to the view that the fun- 

 damental causes of development are to be 

 found not in the egg cell itself but in en- 

 vironmental stimuli and in the interaction of 

 the various parts. Driesch in particular re- 

 gards the egg, or any cleavage cell, as a "har- 

 monic equipotential system," that is, any part 

 is capable of any fate, and its actual fate is 

 determined by its relation to other parts; in 

 the striking phrase of Driesch, "The fate of 

 a part is a function of its position." We now 



