DEVELOPMENT AND DIFI ERENTI ATION 



eficct, or between one reaction and the next in the chain of 

 processes. 



We can now see how the reactions of nucleus and cytoplasm give 

 a progression of events in the cell. But the progression must be an 



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Fig. 52. — The genes of the nucleus draw upon the cytoplasm for the raw materials 

 of their own reproduction and action. The products of this action pass back into 

 the cytoplasm, where these products may have only a short life or may further 

 multiply themselves by self-reproduction, in association with nucleic acid, according 

 to whether the composition of the cytoplasm be fivourablc. hi so doing they 

 change the composition of the cytoplasm and thereby affect the fate of the further 

 gene products which pass into the cytoplasm, hi this way a constant nucleus can 

 be associated with a changing cytoplasm, and hence with a differentiating phenotype, 

 for the cytoplasm is the agent of the nucleus in action. 



hi the diagram the gene products are represented by lines in the cytoplasm, and 

 their varying fates by varyuig lengths of these lines. The products interact with one 

 another and with the plasmagencs (dotted line). The phenotype reflects the totality 

 of the properties of the cytoplasm as shown by the wide arrows. The cytoplasm is 

 also the chamiel by which external materials and conditions reach the nucleus or 

 affect its action (after Mather, 1948). 



orderly one which passes through a regular lite cycle, in the general 

 case, with sexual reproduction. This orderliness is achieved by the 

 adjustment of the hereditary materials, especially of the genes in 

 the nucleus, and it is lost when the genie adjustment is destroyed. 

 One example will illustrate this very general principle. In the 

 morning and evening campions, sub-species ofAIelandrium dioicum, 

 the sexes are separated with XX females and XY males. In the F.^ 



204 



