8 MONTGOMERY — CELLULAR BASIS OF HEREDITY. [Jan. 15, 



with a centrosome, the centre of an aster, at each pole, the chro- 

 mosomes grouped together in a plane midway between the poles 

 and with the long axis of each chromosome coinciding with this 

 plane. Then begins the separation from each other of the halves 

 of each longitudinally split chroniosome and to opposite poles, 

 probably due to the contraction of linin fibres that connect the 

 chromosomes with the centrosomes. Their separated halves come 

 to lie in two groups, one near each centrosome. Finally, each 

 centrosome loses its influence upon the cytoplasm, the radiations 

 around it disappear, each group of chromosomes forms again a 

 rounded nucleus, the cell body constricts between them to form 

 two cells, and as a result there are two cells each with its own 

 nucleus. The remarkable accomplishment is an exactly equal dis- 

 tribution of the chromatin mass to the daughter cells by a very 

 complex mechanical process. 



IV. 



Now is there any particular one of these structures that can be 

 determined as the bearer of hereditary qualities? No one has 

 advocated that it might be a centrosome, and, indeed, there is no 

 reason for considering a centrosome to be any other than a dynamic 

 centre. Such a substance must then be in either the cytoplasm or 

 the nucleus. 



The earlier views were that this particular substance was located 

 in the cytoplasm (Lankester, 1877; Whitman, 1878; Flemming, 

 1882 ; Van Beneden, 1883). But these were hypothetical assump- 

 tions and employed not so much to show a special hereditary sub- 

 stance, as rather to explain the progressive specialization of the 

 cleavage cells. Hereditary traits cannot, moreover, be transmitted 

 by the cytoplasm of the spermatozoon, for in some cases (Echino- 

 derms) the whole cytoplasmic flagellum of the spermatozoon is left 

 outside the egg, and only the head and midbody of the spermato- 

 zoon penetrate the egg in fertilization. There is also the decisive 

 experiment of Boveri, to which we shall recur, showing that the 

 cytoplasm of the egg cell also does not transmit hereditary traits. 



Accordingly the hereditary substance must have its seat in the 

 nucleus, and there is now practically positive evidence that such a 

 germ plasm is the chromatin. The main reasons are as follows : 



(i) The exact distribution of the chromatin in cell division, so 

 that each daughter cell receives just half the amount of chromatin 



