THE VITAL PROPERTIES OF THE CELL 77 



Once for all, it must be admitted that none of the hypotheses 

 which have, up till now, been propounded, are able to furnish us 

 with a satisfactory conception of the causes and mechanical con- 

 ditions of the plasmic movements, and that, therefore, we must 

 confine ourselves to a simple description of observed conditions. 

 This, however, is not to be wondered at, when we consider what 



FIG. 42. Different appearances assumed by drops of oil, which have spread themselves 

 out. (After Verworn, Fig. 11.) 



a number of different opinions are held with regard to the ultimate 

 structure of protoplasm itself (see pp. 18-26), and this must of 

 course affect the explanations tendered of its movements. 



II. Movements of Flagella and Cilia. Unicellular organ- 

 isms, by means of their flagella and cilia, are able to move from 



