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 resdeng stage of the nucleus shows a network or complete 
coil of filaments (chromatin elements) (Fig. 22). 
(4) 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, 1). 
(c) 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). J 
(d) Diviston and separation of the loops.—Each of the loops 
which make up the star divides /ongztudinally 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) Déastrocd.—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). 
(7) 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). 
