340 THE WORLD OF LIFE chap. 



either pole (B, C). The chromatin elements thus immediately be- 

 come arranged in a fixed and regular manner, so that they all come 

 to lie in the equatorial plane of the nucleus, which we may consider 

 as a spherical body." 



Now follows another and even more remarkable stage 

 in the process, which is thus described : 



"The chromatin elements then split longitudinally, and thus 

 become doubled (B), as Fleming first pointed out. It must be 

 mentioned that this splitting is not caused by a pull from the pole 

 threads (spindle threads), which attach themselves to the chromatin- 

 rods on both sides ; the division arises rather from forces acting 

 in the rods themselves, as is proved by the fact that they are often 

 ready to divide, or indeed have already done so, some time before 

 their equatorial arrangement has taken place by means of these 

 threads. 



" The splitting is completed by the two halves being gradually 

 drawn farther apart towards the opposite poles of the nuclear 

 spindle, until they finally approach the centre of attraction or 

 centrosome (D), which has now fulfilled its object for the present, 

 and retires into the obscurity of the cell-substance, only to become 

 active again at the next cell-division. Each separated half of the 

 nucleus now constitutes a daughter - nucleus, in which it (the 

 chromatin) immediately breaks up, and becomes scattered in the 

 form of minute granules in the delicate nuclear network, so that 

 finally a nucleus is formed of exactly the same structure as that 

 with which we started." 



Weismann then discusses and explains the meaning of 

 this strange phenomenon. He says : 



" It is evident, as Wilhelm Roux was the first to point out, that 

 the whole complex, but wonderfully exact, apparatus for the division 

 of the nucleus exists for the purpose of dividing the chromatin 

 substance in a fixed and regular manner, not merely quantitatively, 

 but also in respect of the different qualities which must be contained 

 in it. So complicated an apparatus would have been unnecessary 

 for the quantitative division only. If, however, the chromatin sub- 

 stance is not uniform, but is made up of several or many different 

 qualities, each of which has to be divided as nearly as possible into 

 halves, or according to some definite rule, a better apparatus could 

 not be devised for the purpose. On the strength of this argument 

 we may, therefore, represent the hereditary substance as consisting of 

 different qualities. . . . The statement that this substance is the 

 hereditary substance can, therefore, hardly be considered as an hypo- 

 thesis any longer." ] 



1 The Germ- Plasm, p. 29. 



