THE CELL DOCTRINE. 97 



plasm which seem to have a considerable amount of 

 persistence. The currents in adjacent parts com- 

 monly take similar directions, coursing in a general 

 stream up one side of the hair and down the other, 

 though partial currents also exist which take differ- 

 ent routes; so that sometimes trains of granules 

 .may be seen coursing swiftly in opposite directions, 

 within a twenty-thousandth of an inch of each other; 

 and occasionally opposite streams come in direct 

 collision, and after a longer or shorter struggle, one 

 predominates. The cause of these currents seems 

 to lie in contractions of the protoplasm which bounds 

 the channels in which they flow, but which are so 

 minute that the best microscopes show only their 

 effects, and not themselves. 



Among the lower plants, it is the rule rather than 

 the exception, that contractility should be still more 

 openly manifested at some periods of their existence. 

 The protoplasm of Algse and Fungi becomes, under 

 many circumstances, partially or completely freed 

 from its woody case, and exhibits movements of its 

 whole mass, or is propelled by the contractility of 

 one or more vibratile cilia. 



In illustration of animal protoplasm, Prof. Hux- 

 ley adduces the colorless corpuscles of the blood; 

 which, under the microscope, at the temperature of 

 the body, exhibit a marvellous activity, changing 

 their forms with great rapidity, drawing in and 

 thrusting out prolongations of their substance; and 

 creeping about as if they were independent organ- 

 isms. " The substance which is thus active is a 

 mass of protoplasm, and its activity differs in detail 



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