46 THE ELEMENTS OF STRUCTURE. 



The eventful changes of karyokinesis are as follows : — 



{a) The resting stage of the nucleus shows a network or complete 

 coil of filaments (chromatin elements) (Fig. 19). 



(6) First stage. — As envision begins, the membrane separating 

 the nucleus from the cell substance disappears, and the 

 chromatin elements are seen as a tangled or broken coil 

 (Fig. 22, 1). 



{c) Astroid stage. — The chromatin elements bend into looped 

 pieces, which are disposed in a star, the free ends of the 

 U-shaped loops being directed outwards. Meanwhile a 

 centrosome has appeared and divided into two separating 

 halves, between which a spindle of fine achromatin threads 

 is formed. This seems to form (at least part of) what is 

 called the nuclear spindle. The centrosomes separate until 

 one lies at each pole of the cell, surrounded by radiating 

 ' ' archoplasmic " threads, which become attached to the 

 chromosomes (Fig. 22, 2). 



(d) 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 neighbour. They lie at 

 first near the equator of the cell, but they are apparently 

 drawn, or driven, to the opposite poles (Fig. 22, 2-4). 



(e) Diastroid. — The single star thus forms two daughter stars, 



which separate fuither and further from one another 

 towards the opposite poles of the cell, remaining con- 

 nected, however, by delicate threads (Fig. 22, 3-5). 



(/) Each daughter star is reconstituted into a coil or network for 

 each daughter cell, for the cell substance has been con- 

 stricted meanwhile at right angles to the transverse axis of 

 the spindle. The halves separate in the case of Protozoa, 

 but in most other cases, e.g. growing embryos, they remain 

 adjacent, with a slight wall between them (Fig. 22, 6). 



[g) Each daughter nucleus then passes into the normal resting 

 phase. The spindle disappears, and the centrosomes may 

 also vanish. 



Flemming gives the following summary of karyokinesis : — 



Mother Nucleus Daughter Nucleus 



(progressive changes). (regressive changes). 



a Resting stage. Resting stage, g ^ 



I b Coil. Coil. / I 



Y c Astroid. Diastroid. e \ 



■ ^- d Division of Astroid and its loops ^~ 



(Prophases) (Metakinesis) (Anaphyses). 



We are far from being 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 

 we know the nature of neither. The longitudinal division of each 



