NUCLEAR AND CYTOPLASMIC SYSTEMS 



The nuclear system depends on the protein fibres of its chromo- 

 somes for its mechanical permanence and on desoxyribose nucleic 

 acid for its propagation. It is protected from outside influences by 

 the nuclear membrane in the resting stage, and, during mitosis, it 

 is locked up by its spiralization and by its nucleic acid charge. It 

 consists of linear arrangements of genes, which are of two kinds, 



IG. 53. — The relationships'of position, propagation, and interaction in the cell, of 

 the two types of nucleo-protein responsible for heredity, development and infection. 

 The downward arrows (except within the nucleus) are physiological, the upward 

 evolutionary (after Darlington, 1944). 



or at least lie between two extremes represented by heterochromatin 

 and euchromatin in the cell, and by polygenes and major genes in 

 heredity. The former are the small determinants with the simple 

 products; the latter are the large determinants with the complex 

 products. Between the two there are doubtless intermediates, tran- 

 sitional forms which are also transitory and, therefore, elusive. It is 

 natural to suppose that the more complex is built up from the more 

 simple, by integration of different simples (Fig. 53). Later we shall 



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