575.1 4o6 



591.16 



V. 



The Facts of Chromosome-reduction versus the 

 Postulates of Weismami.' 



IT is advisable that any new observations which bear upon the data 

 supporting a popular theory should be widely known. The 

 present article calls attention to the consequences which arise from 

 the latest contribution to our knowledge of the maturation processes 

 in sexual cells. As is well known, Weismann in the " germplasm " 

 theory regarded the nuclear chromatin as the hereditary substance, 

 while the chromosomes which always appear during nuclear division, 

 in numbers that are fixed for each species, were supposed to contain 

 equal quantities and qualities of hereditary units — "ids." In conse- 

 quence of this hereditary equivalence of the cells, it is necessary that 

 some reduction in the chromatin, the hereditary substance, should take 

 place before fertilization can occur, since the essential process in 

 fertilization is generally regarded as an interchange of half the here- 

 ditary substance of each of the conjugating cells. Weismann (i) was 

 thus led to postulate as necessary the existence of peculiar divisions, 

 in which this chromosome-reduction, or halving of the hereditary 

 substance, could be brought about. The "Reductions-theilungen" or 

 reduction-divisions as Weismann conceived them, consisted of final 

 nuclear divisions during the formation of the sexual elements, in 

 which half the original chromosomes passed unsplit directly to the 

 poles, as in fig. i. 



Now, although it has been subsequently shown that such a 

 process, if it exists at all, is not of general occurrence either among 

 animals or plants, it is still maintained by Hacker (2) that the 

 reduction postulated by Weismann may really be carried out, but in 

 another way. It is agreed now among all observers, zoologists and 

 botanists alike, that a halving of the number of chromosomes does 

 actually take place, apparently universally before the maturation of the 

 sexual elements. But this merely numerical reduction is not brought 

 about by any nuclear division, it always follows a remarkable resting- 



1 Readers unfamiliar with the terms used in this paper will find an explanation 

 of them, as well as a statement of the facts then known, in Mr. M. D. Hill's articles 

 on "Cell-Division " in Natural Science, vol. iv., pp. 38 and 417, especially p. 425 

 (Jan. and June, 1894). — Ed. Nat. Sci. 



