12 



THE ANIMAL CELL. 



[CH. ii. 



The Attraction Sphere. 



Recent 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 attrac- 

 tion sphere. The nucleus with 

 its contorted filament of chro- 

 moplasm is represented, 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. n). It has been thought that the centrosome gives 

 the primary impulse to cell-division, but a few cases have been 

 described in which the nucleus divides before the centrosome, and 

 it is now agreed that the division of chromatin and centrosome are 

 two parallel events, the causal relation between which is not known. 



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 excretion, or the getting rid of waste material. But the 

 most obvious physiological characteristic of most cells is their 

 power of movement. 



When an amoeba is observed with a high power of the 

 microscope, it ie found to consist of an irregular mass of proto- 

 plasm containing 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 



