CHAPTER IV 

 THE NUCLEUS 



It is now half a century since the modern period of 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 

 have to many outstanding biological problems, including that of heredity, 

 it is largely in nuclear phenomena that cytological interest has con- 

 tinued to center throughout the period. The most striking of these 

 phenomena form the subjects of several subsequent chapters: at this 

 point we shall consider the nucleus only as it appears in the "resting" 

 cell, i.e., in the cell not undergoing division. 



Occurrence. The most conspicuous, and in some respects the most 

 important of the cell organs is the nucleus. Whether or not we shall say 

 that every living cell contains a nucleus will depend upon what we are to 

 include under the term. If the chromatin or chromatin-like substances, 

 no matter whether distributed throughout the cell in the form of granules 

 or aggregated to form a well defined organ, be regarded as constituting a 

 nucleus, then it follows that all plant and animal cells normally have 

 nuclei. If, however, as certain protozoologists prefer, the term nucleus 

 be employed only with reference to a distinctly delimited organ, we must 

 regard those lowly organized cells with scattered chromatic material as 

 devoid of nuclei, although they possess, as all cells apparently do, material 

 which performs at least the nutritive functions of a nucleus. This latter 

 type of organization, which is found in certain members of the Protozoa 

 and Bacteria, and also some Cyanophycese, will be discussed later on in 

 connection with nuclear structure (p. 66) and cell-division (Chapter X). 

 In myxomycetes, where simple and primitive conditions might be expected, 

 Jahn (1908, 1911) and Olive (1907) have demonstrated the presence of 

 definite nuclei showing mitosis and the phenomenon of chromosome 

 reduction. 



General Characters. The vast majority of cells have one nucleus 

 each. A few exceptions may be noted. In tapetal cells, laticiferous ves- 

 sels, the internodal cells of Characeae, and certain other cells there are 

 often several nuclei arising by the division of one. In the Siphonese among 

 green algae (Fig. 14, V) and the Phycomycetes among fungi there are no 

 cross walls in the filamentous and much branched vegetative body, so that 



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