MATURATION OF EGG-CELL 21 



centres in the chromosomes. The doubUng of their number 

 during the development of the primitive germ-cell is very re- 

 markable. The reason for it may be found, as Weismann has 

 suggested, in the fact that it enables the number of variations 

 produced by amphimixis, or the mingUng of the parental ele- 

 ments, to be enormously increased. The successive divisions 

 preceding maturation would, in the event of the number of 

 chromosomes not being doubled, have the efEect of reducing the 

 chromosomes to one-fourth of the normal number possessed by the 

 species. As it is, the possible combinations of the chromosomes 

 are very greatly increased through the doubling which precedes 

 their ultimate reduction ; for it must be observed that the next 

 result of maturation, as far as the chromosomes are concerned, 

 is their reduction by half. Thus, in the hypothetical case before 

 us, the number of chromosomes in the original germ-ceU is four ; 

 after the maturation the ovum contains but two (Fig. 2, F). 



As for the polar bodies which are " expelled," so to speak, 

 from the egg-ceU during its maturation, there seems little doubt 

 that they are abortive ova, incapable of attracting the sperm-cell, 

 and destined only to disappear. The difference between the 

 maturation of the sperm-cell and that of the ovum is that, 

 whereas the four spermatozoa which result from the successive 

 divisions of the spermatocyte (corresponding to the ovocyte) 

 remain complete, and potentially capable of fertilising the egg, 

 three of the products of the ovocyte are eliminated in the form 

 of polar bodies, and only one — the mature ovum — is capable of 

 development. The reason for the impotency and essential dis- 

 appearance of the polar bodies is to be found in the fact that 

 they lack the protoplasmic substance necessary to maintain life. 



The process of maturation in the case of the spermatozoon is 

 much the same as in that of the ovum. The primitive germ-cell 

 goes through a period of growth, and develops into the sperma- 

 tocyte. During this development the number of chromosomes 

 is doubled. The spermatocyte then undergoes two successive 



