36 MITOSIS. 
astrosphere; it contains in its centre a minute spot called a 
centrosome. Theastrosphere and the chromatin appear to 
play important parts in the process of cell-division. 
A cell reproduces 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 éudirect or 
mitotic division the nucleus undergoes division by m¢osis. 
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 
doubling 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.—D1aGRam oF Mitosis. (After FLEMMING. ) 
4. Loops migrate to each Centrosome. 
5- Cell commences to divide. , 
6. Division complete. Re-formation of 
Nuclei. 
1. Chromatin Loops. 
2. Loops split and Centrosome divided. 
3. Centrosomes have diverged and loops 
are at equator. 
