48 THE ELEMENTS OF STRUCTURE. 



following : There is a striking similarity in most cases, and 

 the nucleus plays an essential part in the process. The 

 dividing nucleus usually passes through a series of complex 

 changes known as karyokinesis or mitosis, and these are 

 much the same everywhere, though different kinds of cells 

 have their specific peculiarities. Occasionally, however, 

 both in Protozoa and Metazoa, the nucleus divides by 

 simple constriction (direct or amitotic division). This is a 

 quicker process than the other, and occurs especially when 

 there is rapid growth or frequent replacement of cells. 

 Another departure from the ordinary scheme is seen when 

 the nucleus shows a multiple division, while the cell 

 remains undivided. This occurs normally in some marrow 

 cells. 



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). 



(:) Astroid stage. The chromatin elements bend into looped 

 pieces (or chromosomes), which are disposed in a star, lying 

 flat at the equator of the cell, 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 connected, how- 

 ever, 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). 



