106 NUCLEAR DIVISION IN PROTOZOA 



When reproduction by binary fission occurs, division of the nucleus 

 is followed by division of the body of the organism into two parts. When 

 multiplication by schizogony takes place, or when a number of gametes 

 are produced, the nucleus divides into two, these into four, and so on, 

 till the requisite number is reached. The multinucleated organism then 

 buds from its surface a number of daughter individuals. The repeated 

 divisions of the nuclei frequently take place by mitosis, especially when 

 they are multiplying to form gamete nuclei, as in the case of gregarines 

 or coccidia. It sometimes happens that before the spindle of one division 







f/ 







^ 



if.. 



■A 



Fig. 62. — First Nuclear Division in One of a Pair of Associated 



MONOCTST'ID G-REGARINES {MonocysUs SP.) OF THE EaRTH-WORM. (AfTER 



Brasil, 1905.) 



1, 2. Two centrosomes are present, the nucleolus is breaking up, while the chromatin has collected 

 at the centre of the nucleus ( X 900). 



3. The spindle has formed, the nucleus has been extruded, and chromosomes are found at the 

 equator of the spindle { X 9C0) . 



•1. The chromosomes have divided and are passing to the poles of the spindle, where the centro- 

 somes have already divided for the succeeding division ( X 900). 



5. Though the nuclei have not been definitely reconstituted, the spindles for the next division 

 have formed ( X 800). 



has disappeared the two asters and the centrosomes, if these be present, 

 divide again, so that two asters are formed at each end of the spindle. 

 These may separate and form a new spindle between them, so that when 

 the daughter chromosomes reach the pole of the original spindle they are 

 already at the equator of a new one. In this manner very complicated 

 poly-aster figures may arise. Precocious formation of daughter asters 

 while the original spindle is still present has been shown to occur in a 

 gregarine (Monocystis) of the earth-worm by Brasil (1905) (Fig. 62). 

 Very complicated poly-aster figures similarly occur during nuclear 



