DIFFERENTIATION, HEREDITY, SEX 263 



development. This succession of views represents rather the 

 order of their general acceptance as working bases, than of 

 their discovery and individual promulgation. 



In accordance with this historical succession of ideas regarding 

 the nature of the underlying differentiations in development, 

 we may outline briefly three general hypotheses of the causes of 

 differentiation. We may omit, as being now of historical 

 interest chiefly, any further reference to the germ layers as the 

 primary determiners of development, and may begin with the 

 idea of Pfliiger, that the egg and the blastomere group are 

 homogeneous or isotropic throughout, and that the early 

 developmental processes of cleavage are nothing more than 

 indifferent multiplications of similar units, resulting in the 

 formation of blocks out of which later differentiating structures 

 may be built. During the early '90's this view was quite 

 prevalent, and especially favored by such embryologists as 

 Oscar Hertwig and Driesch, who developed it somewhat farther 

 into what has been termed the "cell interaction" hypothesis. 

 According to this hypothesis, while the cells of the blastomere 

 group are essentially similar and equivalent in their poten- 

 tialities (" prospective potency," Driesch), differentiation exists 

 among them by virtue of their relative position in the cell 

 group not through any actual, individual, intracellular 

 differentiation. Stated in the words of Wilson (The Cell, etc., 

 page 415), two sentences of Driesch summarize this view as 

 follows : " The blast omeres of the sea-urchin are to be regarded 

 as forming a uniform material, and they may be thrown about, 

 like balls in a pile, without in the least degree impairing thereby 

 the normal power of development." "The relative position 

 of a blastomere in the whole determines in general what develops 

 from it; if its position be changed, it gives rise to something 

 different; in other words, its prospective value ["prospective 

 significance," Driesch] is a function of its position." 



In itself, then, the cell interaction hypothesis offers no 

 explanation of differentiation or development, for it throws 

 back upon some unknown factor the real cause of differentiation 

 through position. Later investigation, chiefly experimental, has 



