THE ORGANIC CELL 
101 
continuous with the cytoplasm of the cell-body, and the 
chromosomes lie naked in this substance. 
It is a remarkable fact that the number of chromosomes for 
each species of animal or plant is constant. For example, in 
the common mouse there are twenty-four ; in man, the guinea- 
pig, and onion the number is sixteen ; in the grasshopper twelve, 
and so on. These numbers recur regularly in the division of all 
of the cells. It is interesting to know that in all forms arising 
by sexual reproduction the number is even. As will be seen 
later on, the even number results from the fact that one half 
of the chromosomes is derived from each of the parents. 
As regards the fate of the nucleoli, it may be stated that 
the net-knots, which are composed of chromatin, assist in the 
formation of the chromosomes ; while the plasmosomes, or 
true nucleoli, which are devoid of chromatin, disappear. 
(b) The Amphiaster . — In the meantime a spindle-shaped 
body makes its appearance at the site of the original nucleus. 
At either pole of this spindle a star forms, the radiating fibres 
of which are called astral rays — these rays passing into the 
substance of the cell-body. In the centre of each star is found 
a centrosome, and usually surrounding the centrosome a clear 
substance called the centrosphere. According to van Beneden 
and Boveri the centrosome is the ‘ dynamic centre ’ which 
initiates these various changes. The chromosomes now arrange 
themselves in the region of the equator of the spindle, forming 
what is known as the equatorial plate (see diagram, p. 102). 
According to the most recent researches, it would appear that 
the astral rays have the power of forcing their way into the 
nucleus, attaching themselves to the chromosomes, and then, 
by a process of contraction, pulling them to the equator of 
the spindle. The complete form thus produced is called the 
Mitotic figure, and, as will easily be seen by reference to the 
diagram (F), consists of a Chromatic figure, formed of the 
chromosomes, and an Achromatic figure, which includes the 
rest, namely asters and spindle. 
II. Metafhase . — The changes ,so far accomplished have 
been of a preparatory nature, and in the metaphase which 
follows, the most important act in the whole process of cell- 
division occurs. This consists in the longitudinal splitting 
