CH. II.] 



PROTOPLASMIC MOVEMENT 



11 



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

 double (fig. 9). In all probability the centrosome gives the primary 



FIG. 8. A cell (semi-diagrammatic) 

 showing its attraction sphere. 

 In this, as in most cases, the 

 attraction sphere lies near the 

 nucleus. (Schafer.) 



FIG. 9. 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.) 



impulse to cell-division. Some cells, for instance, 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 

 characteristic of most cells is their power of movement. 



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 proto- 

 plasm 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 sub- 

 stance is, as it were, drawn into it. The 

 Amoeba thus comes to occupy a new position, and when this is 

 repeated several times we have locomotion in a definite direction, 

 together with a continual change of form. These movements, when 

 observed in other cells, such as the colourless blood-corpuscles of 

 higher animals (fig. 11), in the branched cells of the cornea and 

 elsewhere, are hence termed amoeboid. The projections which are 

 alternately protruded and retracted are called pseudopodia. 



FIG. 10. Amoebae. 



