48 THE ELEMENTS OF STRUCTURE. 



or mitosis, and these are much the same everywhere, 

 though different kinds of cells have their specific peculi- 

 arities. Occasionally, however, both in Protozoa and 

 Metazoa, the nucleus divides by simple constriction (direct 

 or amitotic division). 



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. 22). 



(b) First stage. As division begins, the membrane separating 



the nucleus from the cell substance disappears, and the 

 chromatin elements are seen as a tangled or broken coil 

 (Fig. 25, i). 



(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. 25, 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. 25, 2-4). 



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

 which separate farther and farther from one another 

 towards the opposite poles of the cell, remaining con- 

 nected, however, by delicate threads (Fig. 25, 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. 25, 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. /(\ 



b Coil. Coil. / 



c Astroid. Diastroid. e 



-^- d Division of Astroid and its loops - 

 (Prophases) (Metakinesis) (Anaphases). 



