260 BIOLOGY 



The methods of maturation and fertilization differ somewhat 

 in different animals and plants, but in all cases where there 

 is the union of the egg with the sperm it is essentially as above 

 described. The normal number of chromosomes is first doubled 

 and then reduced to one-half that which the ordinary cells 

 of the organism originally contained. The mature sperm also 

 contains half of the normal number of chromosomes; and thus, 

 when the egg and the sperm finally fuse, the nucleus of the 

 fertilized egg is always brought back into the original condi- 

 tion with the normal number of chromosomes, which is evi- 

 dently always an even number; see page 85. 



It may seem a little strange that the egg should exclude 

 and throw away as useless such a considerable part of this 

 chromatin material, which must be of such great value. The 

 reason is not difficult to see. If the egg did not throw away 

 some of its chromatin material, it could not combine with the 

 sperm without the chromatin material in the combined egg 

 being doubled in quantity. If, for instance, in the case de- 

 scribed, the egg and sperm should retain their normal number 

 of chromosomes, then, after the egg and sperm united, the 

 nucleus of the fertilized egg would contain eight instead of 

 four, and all of the subsequent cells would necessarily contain 

 eight. If the process were repeated at the next reproduction 

 the number would again double and thus the amount of the 

 chromatin material in each cell would become greater, genera- 

 tion after generation. To keep the number of chromosomes 

 the same in successive generations, both the sperm arid the 

 egg throw away some of their chromatin to make room for 

 an equal amount brought in by the other cell at fertilization. 

 Why the number is first doubled before being reduced is not 

 clear. 



THE PURPOSE OF THE UNION OF THE SEXES 



Since sex union is almost if not quite universal among 

 animals and plants, it is evident that the process must be 

 one of very great significance. One of its purposes is very 



