THE STORY OF LIFE'S MECHANISM, 



CELL DIVISION OR KARYOKINESIS. 



We will begin with a cell in what is called the 

 resting stage, shown at Fig. 23. Such a cell has 

 a nucleus, with its chromatin, its membrane, and 

 linin, as already described. Outside the nucleus 

 is the centre-some, or, more commonly, two of 

 them lying close together. If there is only one 

 it soon divides into two, and if it has already 

 two, this is because a single centrosome which 

 the cell originally possessed has already divided 

 into two, as we shall presently see. This cell, 

 in short, is precisely like the typical cell which 

 we have described, except in the possession 

 of two centrosomes. The first indication of 

 the cell division is shown by the chromatin 

 fibres. During the resting stage this chro- 

 matin material may have the form of a thread, 

 or may form a network of fibres (see Fig. 27). 

 But whatever be its form during the resting 

 stage, it assumes the form of a thread as the cell 

 prepares for division. Almost at once this thread 

 breaks into a number of pieces known as chromo- 

 somes (Fig. 28). It is an extremely important fact 

 that the number of these chromosomes in the 

 ordinary cells of any animal or plant is always 

 the same. In other words, in all the cells of the 

 body of animal or plant the chromatin material 

 in the nucleus breaks into the same number of 

 short threads at the time that the cell is preparing 

 to divide. The number is the same for all animals 

 of the same species, and is never departed from. 

 For example, the number in the ox is always 

 sixteen, while the number in the lily is always 



