332 HEREDITY AND ENVIRONMENT 



from any portion of the ectoderm, or outer 

 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 ves- 

 sels 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 animal 

 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 differentia- 

 tion," implying that differentiation is depend- 

 ent upon the stimuli which come from sur- 

 rounding parts. On the other hand if the 

 differentiation has already begun before the 

 relation of a part to surrounding parts has 

 been changed, it may continue to differentiate 

 as if no change of position or relation had 



