cd . 



184 FOOT-NOTES TO EVOLUTION. 



that no reducing division in Weismann's sense takes 

 place, though the actual number of the chromosomes is 

 also reduced. 



Boveri has shown for the egg and Brauer for the 

 sperm that the tetrads arise by a double, longitudinal 

 splitting of the chromatin filament which later breaks 

 into two segments. Thus abed would again represent 

 the unsegmented filament, a-b-c-d the individual chromo- 

 somes, and -, -,, -, -their splitting longitudinally in or- 

 a b e d 



dinary division. In the maturation of the ^g% and in 



spermatogenesis, however, the thread segments into ab, 



. ab ab ed 

 cd, and splits twice longitudmally mto —r— 7. —1 



two tetrads of B in Fig. 11. The reduction of chromatin 

 here is only a reduction in mass and not a qualitative 

 one, in Weismann's sense, as in the Crustacea and in- 

 sects. In Ascaris the actual reduction in number of 

 chromosomes takes place in the nucleus previous to the 

 maturation divisions of the ovocyte and spermatocyte 

 respectively. In Cyelops the formation of the tetrads is 

 merely a pseudo-reduction, the actual reduction taking 

 place in the second division which gives rise to the ma- 

 ture egg on the one hand, or to the spermatids, which 

 develop into the spermatozoa, on the other. 



One fundamental fact is clear in these divergent 

 accounts. The number of the chromosomes is reduced 

 in both sorts of the germinal cells as a preliminary to 

 their union. Whether there is likewise a qualitative dis- 

 tribution of the chromatin elements remains for future 

 investigation to decide. 



From the facts of ordinary cell division we have seen 

 the probability of the hypothesis that the chromatin of 

 the nucleus is to be regarded as the bearer of hereditary 

 qualities in the cell. The phenomena of fertilization 



