STRUCTURE OF THE NUCLEUS 35 



The process may be purely physical and consist in the transmission of 

 some particular form of molecular motion ; or it may involve chemical 

 changes, such as the secretion of an enzyme or other active substance. 

 Very likely one or the other type of action is employed alternatively, 

 according to the kind of metabolic change or growth-process which has 

 to be induced. 



It has already been stated, that it is not the whole of the nucleus 

 which should be regarded as the vehicle of the idioplasm, but only that 

 portion thereof which gives rise to the chromosomes, namely, the 

 nuclear reticulum. The physiological importance of the other com- 

 ponents of the nucleus is still obscure. The nuclear membrane in all 

 probability represents a plasmatic membrane (comparable to a tonoplast), 

 which regulates the interchange of material between nucleus and cyto- 

 plasm. The nuclear sap presumably serves in the first instance merely 

 as a medium in which the reticulum can be suitably suspended ; it 

 may also to some extent act as a repository of reserve-materials. 

 Opinions differ as to the significance of the nucleoli, which are 

 ordinarily the most conspicuous inclusions of the nucleus. Some 

 biologists hold that the nucleolar substance is a reserve-material 

 which is absorbed by the chromosomes during mitosis, and secreted 

 afresh when the daughter-nuclei are reconstituted. Strasburger like- 

 wise regards the nucleolar substance as a reserve-material, but believes 

 that it serves to replenish the " kinoplasm," especially in connection 

 with the formation of spindle-fibres. Hacker finally maintains that 

 nucleolar material is a by-product of the vegetative metabolism of the 

 nucleus, and that it is excreted into the cytoplasm,, during mitosis, 

 either in the solid form or in a state of solution. 14 The function of the 

 nucleolus is likely to remain a matter of uncertainty, until our know- 

 ledge of all the chemical changes attendant upon mitosis is more exact 

 than it is at the present day. 



Whereas it was formerly supposed that nuclei might become 

 differentiated de novo from cytoplasm (by so-called " free nuclear 

 formation"), it is now established beyond fear of contradiction that these 

 bodies never arise otherwise than by division [or fusion] of pre-existing 

 nuclei. All the nuclei, therefore, which are present in the adult 

 organism are the lineal descendants of the nucleus contained in the 

 germ-cell. It is quite exceptional for a germ-cell to contain more 

 than a single nucleus (instances are furnished by uredospores and by 

 the ascospores of Pertusaria). The detailed discussion of nuclear division 

 must be postponed until cell-division comes to be considered, because 

 in all ordinary uninucleate cells the two processes are correlated with 

 one another in a peculiar manner. 



The structure of the nucleus is extraordinarily uniform not only 



