132 THE STORY OF LIFE'S MECHANISM. 



With this understanding we can, at least, in 

 part understand the purpose of fertilization. As 

 we shall see later, it is very necessary in the 

 building of the living machine for each indi- 

 vidual to inherit characters from more than 

 one individual. This is necessary to produce 

 the numerous variations which contribute to 

 the construction of the machine. For this pur- 

 pose there has been developed the process of 

 sexual union of reproductive cells, which intro- 

 duces into the offspring chromatic material from 

 two parents. But if the two reproductive cells 

 should unite at once the number of chromo- 

 somes would be doubled in each generation, 

 and hence be constantly increasing. To pre- 

 vent this the polar cells are cast out, which 

 reduces the amount of chromatic material. The 

 union of the two pronucleii is plainly to produce 

 a nucleus which shall contain chromosomes, and 

 hence hereditary traits from each parent and 

 the subsequent splitting of these chromosomes 

 and the separation of the two halves into daughter 

 nucleii insures that all the nucleii, and hence all 

 cells of the adult, shall possess hereditary traits 

 derived from both parents. Thus it comes that, 

 even in the adult, every body cell is made up of 

 chromosomes from each parent, and may hence 

 inherit characters from each. 



The cell of an animal thus consists of three 

 somewhat distinct but active parts the cell 

 substance, the chromosomes, and the centrosome. 

 Of these the cell substance appears to be handed 

 down from the mother; the centrosome comes, 

 at least in some cases, from the father, and the 



