424 HEREDITY AND DEVELOPMENT 



and by their relations to neighbouring bands, and gradually 

 they, too, differentiate into distinctive kinds of settlements. 

 We can quite well understand that certain interpreters or his- 

 torians would lay emphasis on the fact that, as the originally 

 similar bands of colonists journeyed, they became differentiated 

 in response to the varied environmental conditions and in 

 relation to their neighbours. Their prospective value at any 

 moment is " a function of their locality." This is a far-off 

 image of the non-mosaic theory of development. It is surely 

 conceivable that both interpretations are correct. 



Summary. — According to the mosaic theory, the main mode 

 of differentiation is qualitative nuclear division, which sifts out 

 the various items of the mosaic (the representative particles 

 or primary constituents) into different cells. Thus, if the fer- 

 tilised ovum had the qualities or potential qualities abcxyz, its 

 first four daughter- cells or blastomeres might have the qualities 

 abcxyz, abxyz, abcxy, and abcxz. What each cell becomes is prim- 

 arily determined by the particular contingent of representative 

 particles which possesses it. 



According to the non-mosaic theory, the division of the 

 nucleus is always quantitative — i.e. without any sifting out of 

 particular potentialities — and differentiation is due to the varied 

 relations in which the nuclei find themselves. The prospective 

 value of an embryonic cell, Driesch said, is " a function of its 

 location." Each of the early cells is supposed to have a com- 

 plete set of specific characteristics in potentia ; but some of these 

 remain latent, while others become active, this being determined 

 by the relations of the cell to the whole system of which it forms 

 a part. 



Thus, while the two views agree in attributing to the essential 

 germinal material a specific organisation corresponding to the 

 hereditary qualities, they differ in their picture of what dif- 

 ferentiation implies, the mosaic theory relying on the hypothesis 

 of qualitative division which segregates representative particles, 



