THE FEMALE REPRODUCTIVE ORGANS. 849 



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 a*ssume that a similar change takes place in man. 

 From a biological standpoint the reduction of chromosomes and 

 the loss of the centrosome throw much light upon the necessity of 

 fertilization by the male cell. The spermatozoon before it is ripe 

 undergoes a process of maturation essentially similar to that des- 

 cribed for the ovum. Two cell divisions take place with the for- 

 mation of four spermatozoa, each of which, however, is a functional 

 spermatozoon. In the process of division the number of chromo- 

 somes in each cell is reduced 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 sub- 



Ovarian egg. 



First polar body. 



Mature egg f 4 * Abortive ova resulting 



from division of first 

 polar body. 



Second polar body (abortive ovum). 

 Fig. 270. Schema to indicate the process of maturation of the ovum. (Boveri.) 



stance 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 centro- 

 some by the matured egg is interpreted by Boveri as follows : * This 

 minute structure is the instrument by which the mechanical 

 process of cell division is initiated and completed. Its loss by the 

 matured ovum prevents this cell from further development, but 

 in the act of fertilization the spermatozoon brings into the egg a 

 new centrosome, or causes the formation of a new centrosome after 

 its entrance, and thus immediately starts the process of cell mul- 

 tiplication. From this standpoint the loss of the centrosome 



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

 Jena, 1902. 

 54 



