382 THE WONDER OF LIFE 



which usually come from two different parents, and it 

 is important to understand that the spermatozoon from 

 the one parent and the ovum from the other contribute 

 the same number of chromosomes, except in certain very 

 interesting cases where half the spermatozoa have an extra 

 chromosome. Furthermore, anticipating a little, we may 

 notice the ocular demonstration of the fact that when 

 the fertilized ovum divides, each daughter-cell or blastomere 

 receives the normal number of chromosomes, half of which 

 are of maternal and half of paternal origin. This has been 

 followed for several divisions, so that, if the chromosomes 

 are (even in some measure) inheritance-bearers we have a 

 remarkable confirmation of the truth of the prophetic 

 statement which Huxley made in 1878 : 



' It is conceivable, and indeed probable, that every part 

 of the adult contains molecules derived both from the male 

 and from the female parent ; and that, regarded as a mass 

 of molecules, the entire organism may be compared to a 

 web of which the warp is derived from the female and 

 the woof from the male '. 



In the animal kingdom it is usual for cells to have in their 

 cytoplasm, outside their nucleus, a minute body known 

 as the centrosome, which becomes two when the cell is 

 going to divide, and seems to play an important part in 

 the process of division. In the animal ovum the centrosome 

 disappears, and it is part of the process of fertilization that 

 the spermatozoon introduces a centrosome. This divides 

 into two, and these have their role when the egg divides. 

 One passes into each daughter-cell or blastomere. 



Another aspect of fertilization is that the entrance of the 

 spermatozoon, or the reproductive nucleus of the pollen- 

 grain in the case of the flowering plant, does in some way 



