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 " 

 (Wilson, 1900, p. 285). 



The asexual spore-bearing fern-plant has in its cells twice as many 

 chromosomes (2 n) as the sexual prothallus has (n). The spores 

 produced by the fern-plant have n chromosomes ; they develop into 

 a prothallus with n chromosomes ; the prothallus produces sex-cells 

 with n chromosomes ; these undergo no reduction and by their 

 union they restore the number 2 n, which characterises the resulting 

 embryo and the subsequent fern-plant. 



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

 ration-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 theo- 

 retical postulate, but as a fact " (ZellenStudien, iii. 1890, p. 62). 





