SUMMARY AND CONCLUSION 285 



2. A mitosis in the Early Sex-cells 



Whether the progenitors of the germ-cells ever divide amitotically is a question of 

 high theoretical interest. Numerous observers have described amitotic division in 

 testis-cells. and a few also in those of the ovary. The recent observations of Meves 

 ('91), Vom Rath ('93), and others leave no doubt whatever that such divisions 

 occur in the testis of many animals. Vofn Rath maintains, after an extended inves- 

 tigation, that all cells so dividing do not belong in the cycle of development of the 

 germ-cells ('93, p. 164) ; that amitosis occurs only in the supporting or nutritive cells 

 (Sertoli-cells, etc.), or in such as are destined to degenerate, like the " residual 

 bodies" of Van Beneden. Meves has, however, produced strong evidence ('94) that 

 in the salamander the spermatogonia may, in the autumn, divide by amitosis, and in 

 the ensuing spring may again resume the process of mitotic division, and give rise to 

 functional spermatozoa. On the strength of th^se observations Flemming ('93) him- 

 self now admits the possibility that amitosis may form part of a normal cycle of devel- 

 opment. 1 



H. SUMMARY AND CONCLUSION 



The one fact of maturation that stands out with perfect clearness 

 and certainty amid all the controversies surrounding it is a reduction 

 of the number of chromosomes in the ultimate germ-cells to one-half the 

 number characteristic of the somatic cells. It is equally clear that this 

 reduction is a preparation of the germ-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, while 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 followed 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. 



Two general types of maturation may be distinguished according 

 to the manner in which the primary chromatin-masses divide. In one, 



1 For more recent literature on this subject see Meves, Zelltheilung, in Merkel and Bon- 

 net's Ergebnisse, VIII., 1898. 



