104 
THE OBGANIC CELL 
Put briefly, it may be stated that in some cases the chromo- 
somes do not split longitudinally until they have arranged 
themselves in the equatorial plane of the spindle — in other 
cases the splitting occurs in the spireme stage, or even earlier, 
but such exceptions do not in any way affect the central fact 
that the ‘ chromatin network is converted into a thread, 
which, whether continuous or discontinuous, splits throughout 
its entire length into two exactly equivalent halves.’ This 
essentially important phenomenon was discovered by Flemming 
in 1880. It is very noteworthy, that the nuclear division 
always shows this mathematical equality, whether the division 
of the cell-body is equal or otherwise. 
III. Anaphases . — The daughter chromosomes, which result 
from the longitudinal splitting of the original ones, diverge 
to opposite poles of the spindle. As they separate, they are 
seen to be connected by fibres called interzonal fibres. These 
are believed by some to have a special origin and function, 
and to be quite distinct from the ones forming the spindle. 
They almost invariably have a row of deeply staining bodies 
in the plane of the equator called the mid-body. It is 
interesting to note that, in the Mitosis of plant-cells, the mid- 
body is very marked. 
IV. Telophases . — The entire cell now divides along the plane 
of the equator of the spindle into two daughter- cells, in each 
of which a daughter nucleus is formed from the chromosomes 
it contains. 
The Mechanism of Mitosis 
Van Beneden’s hypothesis of fibrillar contractility is, up 
to the present, the most satisfactory explanation of the 
phenomena of Mitosis. To quote his own words : ‘ In our 
opinion all the internal movements that accompany cell- 
division have their immediate cause in the contractility of 
the protoplasmic fibrillse, and their arrangement in a kind of 
radial muscular system, composed of antagonising groups. 
In this system the central corpuscle (centrosome) plays the 
part of an organ of insertion. It is the first of all the various 
organs of the cells to divide, and its division leads to the 
