12 



THE ANIMAL CELL 



[CH. 11. 



The Attraction Sphere. 



Kecent research has shown that, in addition to the nucleus and 

 protoplasm, most if not all living cells contain another structure ; it 

 consists of a minute particle called a " centrosome," which has an 

 attractive influence on protoplasmic fibrils and granules in its 

 neighbourhood, the whole appearance produced being called an 

 attraction sphere (fig. 10). 



FIG. 10. A cell (white blood-cor- 

 puscle) showing its attraction 

 sphere. In this, as in most 

 cases, the attraction sphere lies 

 near the nucleus. (Schafer.) 



FIG. 11. Ovum of the worm Ascaris, 

 showing a twin attraction sphere. 

 The nucleus with its contorted 

 filament of chromoplasm is repre- 

 sented, but the protoplasm of 

 the cell is not filled in. (v. 

 Beneden.) 



It is most prominent in cells which are dividing or about to 

 divide. The centrosome, and then the attraction sphere, become 

 double (fig. 11). In all probability the centrosome gives the primary 

 impulse to cell-division. Some cells, like the giant cells of red 

 marrow, contain numerous centrosomes. 



Protoplasmic Movement. 



A cell possesses the power of breathing, that is, taking in oxygen ; 

 of nutrition, of building itself up from food materials ; and of excre- 

 tion, or the getting rid of waste material. But the most obvious 

 physiological characteristic of most cells is their power of move- 

 ment. 



When an amoeba is observed with a high power of the micro- 

 scope, it is found to consist of an irregular mass of protoplasm con- 

 taining one or more nuclei, the protoplasm itself being more or less 

 granular and vacuolated. If watched for a minute or two, an 

 irregular projection is seen to be gradually thrust out from the main 

 body and retracted ; a second mass is then protruded in another 

 direction, and gradually the whole protoplasmic substance is, as it 



