MITOSIS. 



astrosphere ; it contains in its centre a minute spot called a 

 centrosome. The astrosphere and the chromatin appear to 

 play important parts in the process of cell-division. 



A cell reprojl^es itself by binary fission (see Chapter 

 v.), and there are two types of cell-division, according to 

 the behaviour of the nucleus. In both types, the nucleus 

 first divides into two, the cytoplasm following. In the 

 direct or amitotic division the nucleus merely constricts into 

 two equal parts without special changes. In the indirect or 

 mitotic division the nucleus undergoes division by mitosis. 

 This is the most usual method of cell-division. 



The changes, in a typical instance (see Fig. 12), may 

 be summarised as follows : — 



1. The chromatin network breaks up into a number of chromo- 



somes, usually elongated rods of chromatin. 



2. The chromosomes split down the centre into halves, thus 



doublin*^ their number, and the astrosphere divides into two 

 parts which move to opposite ends of the cell. 



3. The nuclear membrane, nucleoli, and nuclear sap disappear 



and the chromosomes lie in the cytoplasm. 



4. Half of the chromosomes migrate to one astrosphere and half 



to the other, in the neighbourhood of which they are aggre- 

 gated into a nuclear network, and formed into fresh 

 nuclei. 



5. The cytoplasm then divides into two, and cell-division is 



complete. 



Fig. 12.- 



-DiAGRAM OF Mitosis. (After Flemming.) 

 I 2 3 



1. Chromatin Loops. 



2. Loops split and Centrosome divided. 



3. Centrosomes have diverged and loops 



are at equator. 



4. Loops migrate to each Centrosome. 



5. Cell commences to divide. 



6. Division complete. Re-formation of 



Nuclei. 



