ANIMAL AND VEGETABLE 103 



CHAPTER VII 



CELL REPRODUCTION 



MITOTIC DIVISION. THE CENTROSOME AND THE 

 ATTRACTION SPHERE 



IN a diagram of a cell (Schafer) the centrosome is 

 shown double and lying near the nucleus. 

 This is a minute particle (centriole), surrounded 

 by a clear area (attraction sphere) and from it 

 radiate into the surrounding protoplasm a 

 number of fine fibrils and dot-like enlarge- 

 ments at intervals. The twin spheres are 

 connected by a spindle-shaped system of delicate fibrils 

 (achromatic spindle), and this duplication invariably precedes 

 the division of a cell into two. 



In the process of division of a cell many changes occur, 

 but it is always " preceded by the division of its attraction 

 sphere, and this again appears to determine the division of 

 the nucleus." These changes are, briefly, as follows : 



" (1) The network of chromoplasm-filaments of the 

 resting nucleus becomes transformed into a sort 

 of skein, formed apparently of one long convoluted 

 filament, but in reality consisting of a number of 

 filaments (spirem) ; the nucleus membrane and 

 the nucleoli disappear, or are merged in the skein. 



" (2) The filament breaks into a number of separate 

 portions, often V-shaped, the chromosomes. . . . 

 As soon as the chromosomes become distinct they 

 are often arranged radially round the equator of 

 the nucleus like an aster. 



