PROTOZOA 



chromosomes (J), so as to combine them into a nucleus for the 

 daughter-cell. The reorganisation of the young nucleus certainly 

 varies in different cases, and has been ill -studied, probably 

 because of the rapidity of the changes that take place. The 

 cytoplasm now divides, either tapering into a " waist " whicli 

 finally ruptures, or constricting by the deepening of a narrow 

 annular groove so as to complete the formation and isolation of 

 the daughter-cells. 



We might well compare the cell-division to the halving of a 

 pumpkin or melon, of which the flesh as a whole is simply 

 divided into two by a transverse cut, while the seeds and the 

 cords that suspend them are each singly split to be divided 

 evenly between the two halves of the fruit ; the flesh would 

 represent the cytoplasm, the cords the linin threads of the 

 nucleus, and the seeds tlie chromatin granules. In this way 

 the halving of the nucleus is much more complete and 

 intimate than that of the cytoplasm ; and this is the reason 

 why many biologists have been led to regard the nuclear seg- 

 ments, and especially their chromatic granules, as the seat of the 

 liereditary properties of the cell, properties which have to be 

 equally transmitted on its fission to each daughter-cell.^ But we 

 must remember that the linin is also in great part used up in 

 the formation of these segments, like the cords of our supposed 

 melon ; and it is open to us to regard the halving in this 

 intimate way of the " linin " as the essence of the process, and 

 that of the chromatin as accessory, or even as only part of the 

 necessary machinery of the process. The halving or direct 

 splitting lengthwise of a viscid thread is a most difiicult problem 

 from a physical point of view ; and it may well be that the 

 chromatin granules have at least for a part of their function the 

 facilitation of this process. If such be the case, we can easily 

 understand the increase in number, and size and staining power 

 of these granules as cell-division approaches, and their atrophy 

 or partial disappearance during their long intervening periods of 

 active cell life. Hence we hesitate to accept the views so 

 conunonly maintained that the chromatin represents a " germ- 



' See Th. Boveri, Enjchnissc neb. d. Konstitution d. chromatischcn Suhstanz dcs 

 Zcllkcrns (1903), for the most recent defence of this view. He lays, however (p. 2), 

 far more stress on the individuality of the segments themselves than on the actual 

 chromatin material thev contain. 



