250 THE EVOLUTION OF CONTINUITY 



The above is the condensed description of karyokinesis 

 with double chromosomes, each loop splitting longitudinally 

 into two. 



In karyokinesis with single chromosomes the process 

 exhibited is the same, except that the loops do not split. 

 In the first variety each dyaster has the same number of 

 chromosomes as the monaster ; in the second each dyaster 

 has half this number. For this reason " reduction of 

 chromosomes " is said to occur when splitting ceases to 

 take place. 



The process of cell-division has, in effect, three stages : 

 the nucleolar, the nuclear, and the protoplasmic. In the 

 first the nucleolus divides, and as a result each half then 

 takes with it the nuclear substance which it controls ; that 

 is, the nucleus then also divides. Then when the nuclear 

 halves have segregated, each draws to it the protoplasm 

 controlled by, or in sympathy with it. 



The whole process is clearly an exhibition of alternating 

 attraction and repulsion. Cell enlargement as a whole is 

 due to attraction ; and it induces or is followed by the 

 mass-repulsion of cell-division. The nucleolar division with 

 which the process starts is due to repulsion, and each repelled 

 half would seem by its attraction exerted on the nuclear 

 chromatin to induce the repulsion causing nuclear division. 

 In the same way it would appear that the mutual repulsion 

 of the protoplasmic " halves " is induced by attraction 

 towards the opposing nuclear halves. Thus, the suggestion 

 offered is that to the attraction exerted by the mutually 

 repellent nucleolar halves nuclear and protoplasmic division 

 are ultimately due. 



In support of this suggestion the following points may 

 be noted : — 



There is the initial disappearance of the nucleolus, and 

 at the same time the appearance at opposite poles of the 

 cell of two achromatic points. It is not unreasonable to 

 conclude that these two achromatic points are the " division- 

 halves " of the enlarged true nucleolus, whose position 

 during the growth of the cell was indicated by a special 

 area of chromatin concentration, capable of demonstration 

 by staining. 



