102 THE STQRY OF THE LIVING MACHINE. 



nucleus and two centrosomes, and this is exactly 

 t.Iic saoie sort of structure with which the process 

 began. 



Viewed as a whole, we may make the follow- 

 ing general summary of this process. The essen- 

 tial object of this complicated phenomena of 

 karyokinesis is to divide the chromatin into equiv- 

 alent halves, so that the cells resulting from the 

 cell division shall contain an exactly equivalent 

 chromatin content. For this purpose the chro- 

 matic elements collect into threads and split 

 lengthwise. The centrosome, with its fibres, 

 brings about the separation of these two halves. 

 Plainly, we must conclude that the chromatin ma- 

 terial is something of extraordinary importance 

 to the cell, and the centrosome is a bit of ma- 

 chinery for controlling its division and thus regu- 

 lating cell division. 



Fertilization of the Egg. This description of 

 cell division will certainly give some idea of the 

 complexity of cell life, but a more marvelous se- 

 ries of changes still takes place during the time 

 when the egg is preparing for development. In- 

 asmuch as this process still further illustrates the 

 nature of the cell, and has further a most intimate 

 bearing upon the fundamental problem of hered- 

 ity, it will be necessary for us to consider it here 

 briefly. 



The sexual reproduction of the many-celled 

 animals is always essentially alike. A single one 

 of the body cells is set apart to start the next 

 generation, and this cell, after separating from the 

 body of the animal or plant which produced it, 

 begins to divide, as already shown in Fig. 8, 

 and the many cells which arise from it eventually 

 form the new individual. This reproductive cell 



