150 GROWTH PRINCIPLES AND THEORY 2 



administration of somatotropin; this is paralleled by a corresponding multiplica- 

 tion of DNS as measured by microspectroscopy (Leuchtenberger, Helweg-Larsen, 

 and Murmanis, 1954). 



The nuclear series are essentially based upon "inner nuclear division" (Heiden- 

 hain, 1923) which subsequently was elucidated as endomitosis by Geitler (1948); 



DNA(Feulgen) 



Fig. 3. Amounts of DNA, in arbitrary units, in individual mouse liver nuclei, as indicated 



by the Feulgen reaction. , adult; , 10 days old. After Pollister, Swift and 



Alfert, 1 95 1. 



that is, duplication of the chromatin material without consecutive spindle for- 

 mation so that either polytene giant chromosomes (as in the salivary glands of 

 dipters) or else nuclear classes with multiple nuclear volume result. Hence, as a 

 general rule, nuclear series represent polyploidy series with a corresponding 

 multiplication of chromosome number and DNA content. The most important 

 limitation of this rule is in the fact that nuclear size may be changed by variation 

 of protein content (Alfert, Bern and Kahn, 1955). Besides polyploidy, this is 

 another of the appearance of classes with diflferent nuclear volume. 



{b) Interaction between nucleus and cytoplasm 



Protein synthesis and hence cell growth appear to be controlled by the genes 

 or, chemically speaking, the DNA of the nucleus, via the ergastoplasm or RNA 

 of the cytoplasm, although the details of this process are hypothetical. This is 

 shown by a wealth of studies reviewed in other parts of this volume. The main 

 steps in this arguinent are: i. Continued life and protein synthesis of the cell are 

 possible only in the presence of the nucleus. — 2. The specificity of proteins, 

 interspecifically as well as intraspecifically in different mutants, blood groups, etc. 

 is determined genetically, as demonstrated by biochemical genetics which shows 



