32 THE FIRST PRINCIPLES OF HEREDITY 



" centrosome," destined to play an important part in the 

 process of division. 



The first stage of the division is initiated by a rearrange- 

 ment of the chromatin into a long thread, which imme- 

 diately breaks up into a number of small pieces, called the 

 " Chromosomes." The number of these chromosomes is 

 the same for all ordinary cells of the organism, and does 

 not vary within any given species. At the same time, the 

 centrosome, which lies just outside the nucleus, has divided 

 into two, each new centrosome becoming surrounded by 

 radiating fibres, which give it a starlike appearance ; 

 hence the name " aster." The centrosomes now separate, 

 each wandering through a quarter of a circle into the 

 opposite direction, the radiating fibres stretching between 

 them. There seems to be no doubt that the centrosome, 

 with its asters, exerts the determining influence in the 

 division of the nuclear contents. Meanwhile the limiting 

 membrane of the nucleus has disappeared, and the chromo- 

 somes have arranged themselves in the equatorial plane 

 between the two asters. The next step effects the halving 

 of the chromosomes. Each of the chromosomes splits up, 

 not across the middle, but lengthwise, so that instead of 

 each single chromosome we get now a pair of them lying 

 alongside each other. We have, therefore, at this stage a 

 double set of chromosomes, one lying close to the other in 

 the equatorial plane. These two sets next separate from 

 each other by moving to the opposite poles towards the 

 centrosomes, each set thus forming a new daughter- 

 nucleus. Finally, the chromosomes lose their threadlike 

 appearance, form again a network, and surround themselves 

 with a new membrane. As, meanwhile, the cell-substance 

 also has divided, we have at last two complete daughter- 

 cells, each exactly the same as the mother-cell with which we 

 started. We see, in fact, that this process of division effects 

 the exact halving of the chromatin substance, so that each 

 daughter-cell receives not only the same number, but also 

 the same kind of chromosomes as the mother-cell had. 



