104 THE PROTOZOA 



though in some cases it is accomplished at an earlier stage ; it takes 

 place in one of two ways, known respectively as equating and re- 

 ducing division. In equating division each chromosome divides into 

 two daughter-chromosomes, a process which, in the finished and 

 perfect karyokinesis of the higher organisms, is effected by a longi- 

 tudinal splitting of the chromosome, and which may be interpreted 

 as a simple division into two of each of the component chromidio- 

 somes (compare Fig. 60). In reducing division, on the other hand, 

 the individual chromosomes do not divide, but are sorted out, half 

 of them going to one pole of the spindle, and eventually to one 

 daughter-nucleus, the other half to the other ; with the result, 

 finally, that each daughter-nucleus has half the number of chromo- 

 somes possessed originally by the parent nucleus. Equating 

 division is the usual type of karyokinesis seen in ordinary cell- 

 multiplicatibn ; reducing division, on the other hand, is seen only 

 in certain phases of the maturation of the germ-cells, as explained 

 in the next chapter. 



In either type of division, whether equating or reducing, the 

 equatorial plate of chromosomes as a whole divides into two 

 daughter-plates, which separate from one another and travel towards 

 the poles of the achromatinic spindle. As the daughter-plates move 

 away from each other, an achromatinic framework appears between 

 them, in which a longitudinal striation or fibrillation is seen in line 

 with, and continuing that of, the achromatinic spindle. Hence the 

 achromatinic spindle as a whole consists now of the older terminal 

 portions passing from the poles to the daughter-plates, and a new 

 median portion passing between the two daughter-plates ; the two 

 terminal portions constitute together what may be termed conveni- 

 ently the " attraction-spindle," the median portion the " separation- 

 spindle." As the daughter-plates travel further apart, the separa- 

 tion-spindle elongates more and more ; the attraction-spindle, on 

 the other hand, becomes shorter, usually to such a degree that the 

 daughter-plates are brought close up to the poles of the attraction- 

 spindle, which consequently is obliterated and disappears. When 

 full separation of the daughter-plates is attained, the separation- 

 spindle breaks down and disappears gradually, the middle part 

 alone persisting in some cases ; the chromatin of the daughter-plates 

 becomes rearranged to form the daughter-nuclei, going through a 

 series of changes similar to those by which the chromosomes arose 

 from the parent-nucleus, but in inverse order. A nuclear mem- 

 brane is formed round each daughter-nucleus, and the process is 

 complete. 



In the Metazoa, direct and karyokinetic division stand out as the 

 sole types of nuclear division, in sharp contrast and without inter- 

 mediate or transitional forms of the process. In Protozoa, on the 



