20 MITOSIS: THE CONSTANCY OF THE CHROMOSOMES 



some body cells may be only two or three times their bulk. But in 

 the egg cells of some of the higher animals and in the vegetative 

 nuclei of pollen grains, for example, the nucleus may be lo* or lo^ 

 the size of the chromosomes {e.g., Malva, Nemec, 1910, Tischler, 

 1922). 



Special genetic conditions may affect the relation of the bulk of 

 the chromosomes to that of the nucleus, for in the male bee with 

 half the number of chromosomes of the female the nuclei are of 

 similar size in corresponding cells (Nachtsheim, 1913, cf. Sclirader 

 and Hughes-Schrader, 1931 ; Torvik, 1931 ; v. Ch. IX). 



This variation in size bears some relation to the variation in size 

 of the cell in the same organism (the Kern-plasma ratio). When 

 different organisms are considered the volume relationship is 

 apparently subject to genetic control (e.g., Tradescantia, Ch. Ill, 

 Fig. 12). Under the same genetic conditions doubling of the size 

 of the nucleus (by doubling the number of its constituent 

 chromosomes) leaves the volume relationship unchanged in most 

 cases (Ch. VIII). 



In almost all resting nuclei are embedded one or more amorphous, 

 spherical or occasionally rod-shaped bodies of higher density and 

 refractiveness than the surrounding ground substance (Fig. 4). 

 These are called nucleoli. They occupy in all not more than one- 

 twentieth to one-fiftieth of the total volume of the nucleus and 

 characteristically lie away from the surface. With appropriate 

 fixation the nucleoli stain deeply with basic dyes [cf. Zirkle, 1928, 

 1 931). They do not, as a rule, give the thymo-nucleic acid reaction 

 with Feulgen's stain which is characteristic of chromosomes (cf. 

 Geitler, 1935, h on Spirogyra), but they may do so in some circum- 

 stances so that the distinction is not absolute. As a rule they 

 break up and gradually disappear as the nucleus begins to divide, 

 i.e., at prophase, and are reconstituted at the beginning of the next 

 resting stage. For these reasons they cannot be supposed to have any 

 genetic significance or any physiological continuity with the chromo- 

 somes as a whole. 



Owing to its greater density the nucleolus may be expelled from 

 the nucleus by centrifuging (Andrews, 1915 ; Nemec, 1929 a). It 

 may then persist in the cytoplasm as long as twenty-seven days, 



