94 NUCLEAR DIVISION IN PROTOZOA 



chromosomes move to opposite poles of the spindle. The central part 

 of the spindle disappears, the chromosomes break up into granules, and 

 with the formation of a nuclear membrane the nucleus is reconstructed. 

 In subsequent divisions the process is very similar, except that a spindle 

 is formed without definite centrosomes or asters (Fig. 52). 



Very similar mitotic divisions of the nucleus were described by Brasil 

 (1905) also in the case of a species of Monocystis of the earth-worm (Fig. 53). 

 In both these instances the nuclear membrane disappears during division, 

 but in other cases the nuclear membrane persists during the whole mitotic 

 division of the nucleus. 



In the case of Actinosjjhcprium eichhonii, the life-history of which has 

 been described in detail by Richard Hertwig (1898) in a classic memoir, 





• -% 



^/ 



\v 



1 2 



Fig. 53. — Nuclear Divisions in Associated Monocystid Gregaeines 

 (Monocystis sp.) of the Earth-Worm (x 900). (After Brasil. 1905.) 



1. First nuclear division, showing centrosomes, spindles, and elongate daughter chromosomes. 



2. Later nuclear divisions in various stages of mitosis. 



very clear examples of mitosis occur. The multinucleate organism, as 

 mentioned above, becomes encysted in a large primary cyst, within which 

 it divides into a number of daughter individuals round which secondary 

 cysts are formed (Fig. 49). Within the secondary cyst a further division 

 into two individuals takes place. The nucleus of each of these divides 

 by mitosis to form two nuclei, one of which degenerates. A second 

 division of the surviving nucleus takes place, and again one of the 

 resulting nuclei degenerates. After this, the two individuals or gametes 

 in the secondary cyst unite and their nuclei fuse. The various nuclear 



