22 MITOSIS: THE CONSTANCY OF THE CHROMOSOMES 



have been called " chromatin nucleoli," but we need not ascribe 

 any special virtue to this persistence ; a rapidity of prophase 

 changes or a greater viscosity of the nucleus, as Belar suggests, may 

 be held responsible. 



2. THE MOVEMENTS OF THE CHROMOSOMES 



(i) Introduction. Nuclei divide by the characteristic process of 

 mitosis in the course of which the whole nucleus, apart from the 

 nucleoh, resolves itself into longitudinally split threads, the chromo- 

 somes. Recent work, particularly on the Protista, has shown 

 that certain features, precisely those of genetic significance, are 

 universal. Mitosis can accordingly be defined by its essential 

 properties as : the separation of the identical halves of the split 

 chromosomes into two identical groups from which two daughter nuclei 

 are reconstituted. Consequently it may also be said that mitosis 

 is the process by which a nucleus gives rise to two daughter nuclei 

 each morphologically equivalent to one another. 



The longitudinal splitting of the chromosomes was first observed 

 by Flemming in 1880. Its physiological meaning was pointed out 

 by Roux in 1883 : if equivalent nuclei can only be produced by 

 such longitudinal splitting then the chromosomes must be 

 longitudinally differentiated in their physiological properties. 

 Experiment has now shown this conclusion to be valid (v. Ch. VIII). 



There are two kinds of nuclear division : simple mitosis and 

 double mitosis associated with the reduction in the number of the 

 chromosomes ; this is merely an abnormality of the simple mitosis, 

 but it is convenient on account of its special character to refer to it 

 as meiosis in contradistinction to mitosis, the normal type which we 

 are now going to consider. 



(ii) Prophase. The prophase of a mitotic division is first 

 indicated by the appearance of coiled, contorted threads which as a 

 rule, but not always, are evenly spaced throughout the whole body 

 of the nucleus. These structures can be observed in living cells 

 as well as after fixation and staining (Plate XVI, cf. Lucas and 

 Stark, 1 931) although in living material the nucleus first appears 

 granular. The granulation is reasonably supposed to represent an 



