48 THE PHYSICAL BASIS OF INHERITANCE 



cells for their subsequent union and a means by which the 

 number of chromosomes is held constant in the species. With 

 a few exceptions the first indication of the numerical reduction 

 appears through the segmentation of the spireme- thread, or the 

 resolution of the nuclear reticulum, into a number of masses 

 one-half that of the somatic chromosomes. In nearly all higher 

 animals this process first takes place two cell-generations before 

 the formation of the definitive germ-cells, and the process of 

 reduction is completed by two rapidly succeeding ' maturation- 

 divisions,' giving rise to four cells, all of which become functional 

 in the male, while in the female only one becomes the egg, and 

 the other three — the polar bodies or their analogues^are cast 

 aside. During these two divisions each of the original chromatin 

 masses gives rise to four chromosomes, of which each of the 

 four daughter-cells receives one ; hence, each of the latter 

 receives one-half the somatic number of chromosomes.* In the 

 higher plants, however, the two maturation-divisions are fol- 

 lowed by a number of others, in which the reduced number of 

 chromosomes persists, a process most strikingly shown in the 

 pteridophytes, where a separate sexual generation (prothallium) 

 thus arises, all the cells of which show the reduced number " 

 (Wikon, 1900, p. 285). 



As Boveri has said : " Thus at some stage or other in the 

 generation-series of the germ-cell there occurs a reduction of 

 the number of chromosomes originally present to one-half, and 

 this numerical reduction is therefore to be regarded, not as a 

 mere theoretical postulate, but as a fact " {Zellen-Studien, iii. 

 1890, p. 62). 



* Suppose the characteristic number of chromosomes for the species 

 to be 12, the primitive germ-cells share this ; the number is reduced 

 to 6 ; each is increased to 4, and thus each of the 4 daughter-cells receives 

 6 ; and in fertilisation the number 12 is restored. (12 is reduced to 6 ; 

 6 X 4 = 24 ; -\*- = 6 ; 6 + 6 = 12.) After the reduction from 12 to 6, 

 each of the six is seen to consist of two longitudinally split chromosomes, 

 that is of four parts, forming what is called a tetrad group. 



