THE GERM-CELLS 37 



any given species. Before, however, the germ-cells are 

 ready to unite in the act of fertilization, they undergo 

 certain changes, which have the effect of leaving them with 

 only half the number of chromosomes they had previously. 

 This process in the ovum is called its " maturation," 

 while the same end is achieved for the spermatozoon during 

 its development — i.e., during the process of spermato- 

 genesis. 



[a) Ovum. 



In 1875 Biitschli showed that the small polar bodies, 

 which had been observed outside the ovum as far back as 

 1824, resulted from the division of the egg-nucleus itself. 

 After it had further been established in 1883 by E. van 

 Beneden for the round- worm of the horse {Ascaris megalo- 

 cephala), and later for most other animals and plants, that 

 the sex-nuclei of ovum and spermatozoon contain only 

 half the number of chromosomes that are characteristic 

 for the cells of the parent-body, the connection between 

 these two phenomena became gradually cleared up, so 

 that Weismann was finally enabled to formulate his 

 own now generally accepted view, that the matura- 

 tion of the ovum has no other purpose than to effect 

 the reduction of the chromosomes to half their original 

 number. 



But this " reducing division " is not so simple as just 

 suggested, because, before the reduction of the chromo- 

 somes, a doubling of them first occurs, so that, in order to 

 get ultimately the reduced number, the division has to 

 take place twice. 



We have, to take an instance, an egg-cell with four 

 chromosomes before maturation. This egg-cell, as it 

 ripens, grows larger, and doubles its number of chromo- 

 somes, having now eight instead of four. Now, in the 

 first instance, half the number of these eight chromosomes 

 — i.e., four — are removed from the mother egg-cell alto- 

 gether by a process of division, as described above, under 



