70 GENERAL PRINCIPLES OF ZOOLOGY. 



The first step in the karyokinetic formation of the daughter 

 nuclei is the division of the chromosomes, which is usually com- 

 pleted in the equatorial plate (division of the equatorial plate), but 

 occasionally may be completed at an earlier stage. The division 

 is an accurate halving (fig. 22, b). The two halves of a mother- 

 chromosome, the daughter chromosomes, now travel, under the 

 influence of the spindle-fibres, towards the opposite poles of the 

 spindle. In this way, by a splitting of the equatorial plate, the 

 lateral plates arise, the elements of each uniting and producing 

 the daughter nuclei. The centrosomes remain separate as division 

 organs for the next nuclear division (fig. 22, c, d, e). 



What 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 of the protoplasmic 

 reticulum (fig. 21). When the centrosome divides a double radia- 

 tion (amphiaster) appears. Not only the spindle-fibres but the 

 protoplasmic rays extend from the daughter chromosomes. Since 

 the arrangement and degree of development of the protoplasmic 

 radiations stand in certain relation to the phases of cell division 

 we must recognize in them the expression of the effective forces 

 (apparently contractile) in the protoplasm which cause cell 

 division. 



Between these two extreme cases of direct and indirect division are 

 all possible transitions which show how the mechanism of nuclear divi- 

 sion has been completed step by step, first, by the fibrous arrangement of 

 the nuclear reticulum (spindle structure) ; second, through the develop- 

 ment of the centrosome by which the division obtains an influence on the 

 protoplasm ; and third, by the development of the chromosomes. In 

 reference to the latter 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 between the daughter cells. 

 This is accomplished by mitosis. 



Nuclear Fragmentation is to be distinguished from direct division ; by it 

 the nucleus becomes broken up into numerous parts which alone cannot 

 live and as a rule degenerate. A typical example is afforded by the 

 breaking up of the macron ucleus during conjugation in the Infusoria 

 (fig. 146). 



Multinuclearity, Multicellularity. Nuclear division and cell 

 division commonly constitute a well-arranged mechanical process, 



