258 BIOLOGY 



of the animal. Whether the centrosome that is brought in 

 by the sperm and that which comes from the egg have any- 

 thing to do with the subsequent history of the fertilized egg, 

 is uncertain. In some cases it is certain that the (centrosome 

 of the original egg disappears, and the only one that remains 

 is the one brought in by the sperm. In plants, as we have 

 already learned, there are no centrosomes at all, and from 

 these facts it would seem to follow that the centrosome can 

 not have very much to do with the process of fertilization. 



From the facts given it is evident that the fertilized egg con- 

 tains material from both parents. The female parent furnishes 

 the bulk of the food in the egg upon which the young is to be 

 nourished; and it also furnishes two chromosomes. The male 

 parent has also furnished two chromosomes, and in some cases 

 a centrosome, but none of the food material. The only thing 

 which the two sexes have furnished in common is chromatin 

 material, and it is especially interesting to note that both the 

 male and the female parent furnish chromosomes in equiva- 

 lent amounts. 



Unless an egg is fertilized by a sperm it has no power of 

 subsequent growth. Most of the ordinary cells of the animal 

 body are capable of a certain amount of development, but the 

 egg cell if unfertilized soon dies, undergoes decomposition and 

 disappears. The sperm cell also is unable to undergo any 

 development by itself. Therefore, the fusion of an egg and 

 a sperm is necessary, in this type of reproduction, for the 

 development of a new individual. 



It may sometimes happen that more than one sperm is 

 brought into the vicinity of an egg. When this occurs, in 

 most cases there is some device by which the entrance of 

 more than a single sperm into the egg is prevented. In some 

 kinds of eggs, it is, however, not unusual for more than one 

 sperm to enter the egg, but when this occurs only one of them 

 unites with the egg nucleus, the others having no further 

 function in the process. If in any case more than one sperm 



