THE BUILDING-STONES OF THE ORGANIC WORLD 81 



distributed in the nucleus along the nuclear framework. The 

 first step to the indirect nucleus-division is that the fine chromatin 

 granules arrange themselves into one long much-intertwined coil 

 of thread. Simultaneously characteristic changes appear in the 

 centrosome. It divides in two minute grains which become 

 the centre of a system of most delicate fibres ranged like rays ; 

 these two central bodies lie in the protoplasm like two little suns 

 (fig. 23,0. 



Gradually the distance between them increases, and slowly they 

 wander to opposite poles of the nucleus. During this process 

 the ray-system becomes more and more distinct. The origin of 

 these rays or, as they are usually called, 'spindle-figures' is a 

 much-disputed subject. While some say that they come from 

 the protoplasm, others believe that they arise from the ' linin- 

 frame ' of the nucleus. Probably their origin is different in 

 different cell-species ; they may be formed either by the protoplasm 

 or the nuclear substance, or it may be that both substances 

 participate in their production. 



The next important change affects the nucleus. The long 

 uniform chromatin-thread suddenly breaks up into a definite 

 number of separate sections, the nuclear-loops, or chromosomes. 

 The number of the chromosomes is always the same in all body- 

 cells of the same animal or plant species, and all organisms with 

 bisexual reproduction possess always an even number of 

 chromosomes. I would mention in anticipation that this 

 phenomenon finds its explanation in the fact that one half of the 

 chromosomes is of paternal, the other of maternal origin. For 

 instance, anematode common in the horse, Ascaris megalocephala 

 univalens, possesses in all body-cells only two chromosomes, 

 whilst another of the nematodes, Acaris megalocephala bivalens, 

 has four. The human body-cells have sixteen, Helix pomatia t 

 some Amphibians, the mouse, the lily, and numerous other 

 organisms have twenty-four. In Artemia, a small crustacean, the 

 number of chromosomes reaches one hundred and sixty-eight. 

 The statement that the number of chromosomes is in all body- 

 cells of the same animal species always the same expresses a 

 rule to which one species of cells, the mature male and female 

 sex-cells, forms a remarkable exception. In these the number of 



