26 EVOLUTION OF LIVING ORGANISMS. 



system of fibres radiating through the cytoplasm from 

 two minute bodies, the centrosomes. Between these 

 centrosomes the fibres join across, forming a spindle. 

 The centrosomes can be seen to originate from the nucleus 

 or its neighbourhood, as a single body which divides, the 

 two halves moving to the opposite sides of the nucleus. 

 The chromatic thread now breaks up into a definite 

 number of separate pieces, the chromosomes, which 

 arrange themselves in a circle round the equator of the 

 spindle. Each chromosome now divides into two halves 

 which travel to the opposite ends of the spindle. There 

 they join together to form a thread ; the thread breaks 

 up into granules, the system of fibres disappears, and 

 thus a new nucleus is reconstituted similar to the resting 

 nucleus of the original cell. A division of the cell-body 

 then yields two nucleated cells (Fig. 1). As a rule the 

 centrosome persists to give rise to that of the next 

 division. Now it is important to notice the continuity of 

 substance during this process of division. Cytoplasm, 

 limn, centrosome, and chromatin are all parcelled out to 

 the two daughter cells ; above all each daughter nucleus 

 receives the same number of chromosomes, and apparently 

 exactly the same amount of chromatin. The number of 

 chromosomes is approximately constant in each species, 

 but differs widely even among allied forms. 



Although the process of karyokinesis may differ in 

 detail in various forms, yet it is essentially the same in all 

 plants and animals. 



Why, it may be asked, has the cell structure been uni- 

 versally adopted ? To this question we can give no more 

 definite answer than this : that there seems to be some 

 proportion of mass to surface, and of nucleus to cytoplasm, 

 within which it is necessary for the protoplasm to keep if 

 metabolism is to be satisfactorily carried out. The size of 

 cells varies considerably ; much more than the size of 

 nuclei. It bears no direct relation to the size or com- 

 plexity of the organism to which the cell belongs. A cell 



