544 CELL-DIVISION ciiAr. 



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

 cases two astrospheres and centrosomes are found in each 

 cell (b). 



Cell-division. 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. 136). 



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 chromo- 

 somes (b, cJir), the number of which appears to be constant 

 in any given species of animal or plant, although it may 

 vary in different species from two to twenty-four or more. 

 Soon after this the nuclear membrane and the nucleoli dis- 

 appear (b, c), and the nucleus is seen to contain a spindle- 

 shaped body {sp) formed of excessively delicate fibres which 

 converge at each pole to the corresponding astrosphere. 

 The precise origin of this nuclear spindle is uncertain : 

 it may arise either from the nuclear matrix or, more 

 probably, from the protoplasm of the cell : it is not affected 

 by colouring matters. 



At the same time each chromosome splits along its whole 

 length so as to form two parallel rods or loops in close 

 contact with one another (b) : in this way the number of 

 chromosomes is doubled, each one being now represented 

 by a pair. 



The divided chromosomes now pass to the equator of 

 the spindle (d) and assume the form of more or less 

 V-shaped loops, which arrange themselves in a radiating 

 manner so as to present a star-like figure when the cell is 



