236 Heredity and Environment 



layer of the skin, if only the primitive retina, or optic cup, is 

 brought near to this layer; if the optic cup is transplanted from 

 the head to the thorax or abdomen a lens will form wherever the 

 cup comes in contact with the ectoderm. If an embryonic limb is 

 transplanted from its normal position to the middle of the back or 

 belly, it will develop, and nerves and blood vessels will grow into 

 It which would have had very different positions and distributions 

 if the limb had not been there. If one of the first four cleavage 

 cells is separated from the others it may develop into an entire ani- 

 mal though it would have formed only a quarter of an animal if 

 it had remained in contact with the other three-quarters of the 

 egg. All such cases are known as "correlative differentiation," 

 implying that differentiation is dependent upon the stimuli which 

 come from surrounding parts. On the other hand if the differen- 

 tiation has already begun before the relation of a part to surround- 

 ing parts has been changed, it may continue to differentiate as if 

 no change of position or relation had taken place. Thus if a right 

 limb is transplanted 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 dif- 

 ferentiated 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 development have been 

 led to the view that the fundamental causes of development are to 

 be found not in the egg cell itself but in environmental stimuli 

 and in the interaction of the various parts. Driesch in particular 

 regards the egg, or any cleavage cell, as an "harmonic equipoten- 

 tial 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 posi- 



