GENERAL ANATOMY 



61 



elements of each uniting and producing the daughter nuclei. The centro- 

 somes remain separate as division organs for the next nuclear division 

 (iig. 23, c, d, e). 



WTiat further distinguishes the indirect from the direct cell division 

 is the active participation of the protoplasm. The centrosome is the 

 centre of a marked radiation (aster) of the protoplasmic reticulum (fig. 22). 

 When the centrosome divides a double radiation appears, the monaster 

 becomes an ampliiastcr. Not only the spindle-fibres but the protoplasmic 



d e f 



Fig. 23. — Cell division in tlie skin oi Salamandra maculosa (after Rabl). 



rays extend from the daughter chromosomes. Since the arrangement 

 and degree of development of the protoplasmic radiations stand in definite 

 relation to the different phases of cell division we must recognize in them 

 the expression of the forces (apparently contractile) in the protoplasm 

 which cause cell division. 



Between these two extremes of direct and indirect division are transitions 

 wliich show how the mechanism of nuclear division has been completed step by 

 step, first, by the fibrous arrangement of the nuclear reticulum (spindle structure) ; 

 second, through the development of the centrosome by which the division ob- 

 tains an influence on the protoplasm; and third, by the organization of the 

 chromosomes. The irregular division of the chromatin mass in direct division 

 is relatively crude in comparison with the complicated processes involved in the 

 formation and division of the chromosomes. These become intelligible if we 

 regard the chromatin as the controller of the cellular processes and the bearer 

 of heredity (cf. fertilization, infra). The more highly organized the animal, 

 the more its cells have to inherit and the more important it is that the physical 

 basis of heredity should be accurately divided in amount and in quality be- 

 tween the daughter cells. This is accomplished by mitosis. 



