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 ri) as the sexual prothallus has (ri). 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 " (Zellen-Studien, iii. 1890, p. 62). 



