112 THE STORY OF THE LIVING MACHINE. 



thousands of cells which are gradually moulded 

 into the adult. All of these cells will, of course, 

 contain four chromosomes; and, what is more im- 

 portant, half of the chromosomes will have been 

 derived directly from the male and half from the 

 female parent. Even into adult life, therefore, the 

 cells of the animal probably contain chromatin 

 derived by direct descent from each of its parents. 



The Significance of Fertilization. From this 

 process of fertilization a number of conclusions, 

 highly important for our purpose, can be drawn. 

 In the first place, it is evident that the chromo- 

 somes form the part of the cell which contain the 

 hereditary traits handed down from parent to 

 child. This follows from the fact that the chro- 

 mosomes are the only part of the cell which, in the 

 fertilized egg, is derived from both parents. Now 

 the offspring can certainly inherit from each parent, 

 and hence the hereditary traits must be associated 

 with some part of the cell which is derived from 

 both. But the egg substance is derived from the 

 mother alone ; the centrosome, at least in some 

 cases and perhaps in all, is derived only from the 

 father, while the chromosomes are derived from 

 both parents. Hence it follows that the hereditary 

 traits must be particularly associated with the 

 centrosomes. 



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 build- 

 ing of the living machine for each individual to 

 inherit characters from more than one individual. 

 This is necessary to produce the numerous varia- 

 tions which contribute to the construction of the 

 machine. For this purpose there has been devel- 

 oped the process of sexual union of reproductive 



