44 THE ELEMENTS OF STRUCTURE. 



situated in the cell-substance at each pole of 

 the cell, fine " achromatin " threads run into con- 

 nection with the chromatin loops. 



{(T) Division and separation of the loops. — Each of the 

 loops which make up the star divides longitudinally 

 into two, and each half separates from its neigh- 

 bour. They are apparently drawn to opposite poles 

 of the cell. 



(e) Diaster. — The single star is thus drawn out into 

 two daughter stars which separate further and 

 further from one another towards the opposite 

 poles of the cells. 



(/) Each daughter star is reconstituted into a coil 

 in each daughter cell, for the cell-substance has 

 also been halved meanwhile. The halves will 

 ' separate in the case of Protozoa, but in most other 

 cases, as in growing embryos, they will of course 

 remain adjacent with a slight wall between them. 



{g) Each daughter nucleus then passes into the normal 

 resting phase. 

 Flemming gives the following summary of karyokinesis : — 



Mother-Nucleus Daughter-Nucleus 



(progressive changes). (regressive changes). 



a Resting stage. Resting stage, g 



b Coil. Coil. f 



c Aster. Aster (Diaster). e 



^ d Division of Aster and its loop >- 



(metakinesis). s 



(3) It is not in the least likely that we shall ever be able 

 to give even an approximate account of the " mechanism " 

 of cell-division. Rapidly progressive research has disclosed 

 many mysteries, but it does not explain them. The nucleus 

 is resolved into a chromatin framework and an achromatin 

 matrix, but the chromatin threads behave like little in- 

 dividuals. The longitudinal division of each loop shows, as 

 Roux points out, how rigidlyexact is the partition of substance 

 and implied qualities. The "central corpuscles," recently 

 discovered, act like centres of force, and the indescribably 

 fine threads, which pass from these to the chromatin loops, 

 have been credited with contractile powers. The certain 

 fact is that the whole process is vital, and therefore 



