176 



CELLS, TISSLTES, AND ORGANISMS 



reproduction of the nucleolus does not parallel the reproduction of 

 DNA exactly. During interphase, when the DNA doubles, the nucleoli 

 do not double. Rather, they remain constant in number, disappear dur- 

 ing mitosis, and reappear in double numbers at the end of division 

 (Figure 5B). The hypothesis, then, is that physiological reproduction 

 is completed only when the chromosome set has produced two chromo- 

 some sets, complete with nucleoli. 



No one acquainted with contemporary currents of thought in cell 

 biology will be alarmed by the implication that the nucleolus may be 

 a limiting factor in the direction of cytoplasmic synthesis by the 

 nucleus. The idea that the role of the nucleolus is that of a "middle 

 man" between nucleus and cytoplasm (or between DNA and cyto- 



0.0.0 



A. PHYSIOLOGICAL REPRODUCTION 

 (NUCLEAR) 



I = Genetic Material 



0.0 



B. NUCLEOLAR REPRODUCTION 



Figure 5. Speculative models of physiological reproduction in which the 

 reproduction of the nucleus is stressed. A. A general model, according to 

 which the primary genetic material, shown as black bars, reproduces during 

 interphase, but the second nucleus is not completed until the time of divi- 

 sion. B. The nuclear cycle: If the nucleolar equipment is a part of the 

 chromosome complement which may limit its physiological functions, it 

 would follow that the growth potential will not have doubled until the end 

 of division. 



