'PHYSIOLOGICAL UNITS' 77 



lecture of the ids is completely analysed or segregated out, 

 so that each cell ultimately contains only one kind of determinant, 

 the one by which its own peculiar character is determined" 

 (P- 378). 



But how, it may be asked, are the determinants always to find 

 themselves in the right place ? According to Weismann each 

 of them "must be guided through the numerous cell-divisions 

 of ontogemy so that it shall ultimately come to lie in the cell 

 which it is to control. . . . This presupposes that each determinant 

 has from the very beginning its definite position in relation to the 

 rest, and that the germ-plasm therefore is not a mere loose 

 aggregate of determinants, but that it possesses a structure, an 

 architecture, in which the individual determinants have each their 

 definite place " — their relative positions depending, as he says, 

 partly upon heredity and partly upon internal forces or "vital 

 affinities."' But if "the determinants must separate from each 

 other in the course of development, so as to penetrate singly into 

 the cells they are to control," the 'id' must have the power 

 of dividing into daughter ' ids ' of identical and also of dissimilar 

 halves. This second mode, or " differential division," is not a pro- 

 cess that can be observed. Its existence is one of the numerous 

 assumptions made by Weismann in support of his theories, in 

 order to attempt to account for things otherwise inexplicable. 

 Thus, he says, when " one of two sister cells of the embryo gives 

 rise to the cells of the alimentary canal and the other to those 

 of the skin and the nervous system, I infer that the mother cell 

 divided its nuclear substance in a differential way." Yet what has 

 been proved to occur in the ova of the Sea-urchin, of the 

 Amphioxus, and in those of some other animals is directly opposed 

 to this supposition, and shows that their blastomeres are identical 

 in composition. It is not surprising, therefore, that this supposition 

 of a "differential nuclear division" has not been favourably 

 received, especially seeing that Weismann has to make a further 

 postulation of the existence of forces or " affinities " within the 

 ids which control and regulate these identical or differential 

 divisions. 



Speaking generally Weismann holds " that the determinants 



' Though Weismann uses the term " germ-plasm " as that which possesses 

 ■what he calls " an architecture," it should be observed that here, as in other 

 places, he falls back upon the more imposing term " germ-plasm " when he is 

 really speaking of a mere minute granule — one of his 'ids.' 



