882 THE PHYSIOLOGY OF REPRODUCTION. 



and, according to Boveri, also its centrosome. The production 

 of these four cells may be represented, therefore, by a schema of 

 the kind shown in Fig. 279. The details of this process of forma- 

 tion of the polar bodies and of reduction in chromatin material 

 differ somewhat in different animals.* The process has not been 

 followed in the human ovum, but since it occurs in the eggs of all 

 animals with sexual reproduction, so far as they have been studied, 

 it is justifiable to assume that a similar change takes place in man. 

 From a biological standpoint the reduction of chromosomes and 

 the loss of the centrosome, or of the power to produce a centrosome, 

 throw much light upon the significance of fertilization by the male 

 cell. The spermatozoon before it is ripe undergoes a process of 

 maturation essentially similar to that described for the ovum. 

 Two cell divisions take place with the formation of four spermatozoa, 

 each of which, however, is a functional spermatozoon. In the pro- 

 cess of division the number of chromosomes in each cell is reduced 



Ovarian egg. 



First polar body. 



Mature egg f&il Abortive ova resulting * 



from division of first 

 polar body. 



Second polar body (abortive ovum). 

 Fig. 279. Schema to indicate the process of maturation of the ovum. (Boveri.) 



by half. When the matured ovum and the matured spermatozoon 

 fuse, therefore, each brings half the normal number of chromosomes, 

 and the resulting fertilized ovum is a cell with its chromosomes 

 restored to their usual number. The chromatin material is the 

 essential part of the reproductive element. We have reasons to 

 believe, in fact, that it is the substance which has the power of 

 development and which conveys the hereditary structure specific 

 to the animal. The process which causes each element to lose a 

 part of this material before its union with the cell of the opposite 

 sex is a provision by means of which the fertilized egg, from which 

 the offspring develops, shall inherit the characteristics of each parent, 

 without increase in the typical amount of the chromosomes. The 

 loss of the centrosome by the matured egg is interpreted by Boveri 

 as follows:f This minute structure is the instrument by which the 



* For details see Wilson, "The Cell in Development and Inheritance." 

 Second edition, 1900, New York. 



f For a popular presentation see Boveri, "Das Problem der Bef ruchtung. " 

 Jena, 1902. 



