THE CELL AND PROTOPLASM. 97 



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 ordi- 

 nary cells of any animal or plant is always the same. 

 In other words, in all the cells of the body of ani- 

 mal 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 twenty-four. During 

 this process of the formation of the chromosomes 

 the nucleoli disappear, sometimes being absorbed 

 apparently in the chromosomes, and sometimes 

 being ejected into the cell body, where they dis- 

 appear. Whether they have anything to do with 

 further changes is not yet known. 



The next step in the process of division ap- 

 pears in the region of the centrosomes. Each 

 of the two centrosomes appears to send out from 

 itself delicate radiating fibres into the surround- 

 ing cell substance (Fig. 28). Whether these 

 actually arise from the centrosome or are simply 

 a rearrangement of the fibres in the cell sub- 

 stance is not clear, but at all events the centro- 

 some becomes surrounded by a mass of radiating 

 fibres which give it a starlike appearance, or, 

 more commonly, the appearance of a double star, 

 since there are two centrosomes close together 

 (Fig. 28). These radiating fibres, whether arising 

 from the centrosomes or not, certainly all centre 

 in these bodies, a fact which indicates that the 

 centrosomes contain the forces which regulate 



