CHAPTER III 



THE NUCLEUS 



It is now a century since the nucleus was first recognized as a normal 

 and characteristic element of cells, and half a century since a new era in 

 cytology was ushered in by a series of researches revealing the remarkable 

 behavior of the nucleus during the critical stages of the life cycle. 

 Because of the peculiarly intimate relation which this behavior has been 

 shown to bear to many outstanding biological problems, including that 

 of Mendelian heredity, it is largely in nuclear phenomena that cytological 

 interest has continued to center up to the present day. The most striking 

 of these phenomena form the subjects of several subsequent chapters; at 

 this point the nucleus will be considered only as it appears in the "meta- 

 bolic" condition, i.e., when not undergoing division.^ 



General Features of Nuclei. — Whether or not one should say that all 

 protoplasm or all cells are nucleated depends upon what is meant by the 

 term "nucleus." If the chromatic substance, no matter whether dis- 

 tributed throughout the cell in the form of small particles or aggregated 

 in a well-defined organ, be regarded as constituting a nucleus, then it 

 follows that all complete plant and animal cells normally have nuclei. 

 If, however, the term be employed only with reference to a distinctly 

 delimited organ, we must regard those Protista with only scattered 

 chromatic material as devoid of nuclei, although they may contain 

 matter which performs at least certain nutritive functions of a nucleus. 

 The question of the presence of nuclei in bacteria is discussed in Chapter 

 XIII. In nearly all known organisms there are nuclei with roughly the 

 same general type of organization, which indicates that the functional 

 and structural differentiation of protoplasm into cytoplasm and nucleus 

 doubtless occurred at a very early period in the history of the organic 

 kingdoms. 



The number of nuclei present in any mass of protoplasm depends 

 largely upon the bulk of the mass, since within limits a certain ratio of 

 nuclear surface to cytoplasmic volume must be maintained for the 

 proper action of the whole system. A small mass with but one or a very 

 few nuclei may grow into an extensive canocytic body with thousands of 



1 For an exhaustive review of the Uterature on plant nuclei, see Tischler (1921- 

 1922, especially Chaps. I-IV). Von Neuenstein (1914) gives a systematic account 

 of alga nuclei; for recent literature, see Smith (1933). Agar's (1920) book on cytology 

 is chiefly an account of the structure and behavior of the metazoan nucleus. 



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