LESS. VI .CELL-DIVISION 65 



arranged irregularly : the spindle has increased in size and is situated 

 definitely within the nuclear area. 



D, The chromosomes are arranged round the equator of the fully 

 formed nuclear spindle. 



E, The daughter-loops of the chromosomes are passing in opposite 

 directions towards the poles of the spindle, each having a spindle-fibre 

 attached to it. 



F, later stage of the same process. 



G, The chromosomes are now arranged in two distinct groups, one at 

 each pole of the spindle. 



H, The daughter-cells are partly separated by constriction and the 

 chromosomes of each group are uniting to form the network of the 

 daughter-nucleus. 



I, Shows the division of a plant cell by the formation of a cell-plate 

 (c. pi) : the daughter nuclei are fully formed. 



(Altered from Flemming, Rabl, &c. ) 



In the body of some cells and possibly of all there is 

 found a globular body, surrounded by a radiating arrange- 

 ment of the protoplasm and called the astrosphere {s) : it 

 lies close to the nucleus, and contains a minute granule 

 known as the central particle or centrosome {c). In many 

 cells two astrospheres and two or more centrosomes have 

 been found in each cell (b, s). 



The precise changes which take place during the fission 

 of a cell are, like the structure of the cell itself, subject 

 to considerable variation. We will consider what may 

 probably be taken as a typical case (Fig. 10). 



First of all, the astrosphere, with its centrosome, divides (b) 

 and the products of its division gradually separate from one 

 another (c), ultimately passing to opposite poles of the nucleus 

 (d). At the same time the network of chromatin divides 

 into a number of separate filaments called chromosomes (b, c/ir), 

 the number of which appears to be constant in any given 

 species of animal or plant, although it may vary in different 

 species from 2 to 168 or more. Soon after this the nuclear 

 membrane and the free nucleoli disappear (b, c) and the 



F 



