CONTRACTILE TISSUES 55 



the shape which presents the minimum surface. Strong 

 irritation of the ameba causes it to assume this form. 

 It is in fact a general law of contraction that the elements 

 acting approach a spheric form as a geometric limit. 

 When the ameba begins to put out processes and resume 

 movement after a period of intense contraction it is to 

 be supposed that the pressure directed inward at certain 

 points has diminished; thereupon the pressure at other 

 places is no longer counterbalanced and risings of the 

 surface occur in the areas of lowered resistance. 



Ameboid movement is exemplified in the bodies of the 

 higher animals, including our own, by cells found in the 

 blood and called leucocytes. The word means " white 

 cells" and has been applied because these cells are 

 contrasted with the much more numerous red corpuscles 

 of the blood by their lack of pigment. Under favorable 

 conditions many of them change their forms in a manner 

 which is highly suggestive of the free-living amebse and 

 it was maintained at one time that the leucocytes were 

 parasites rather than normal body cells. It is now 

 established that they are both normal and valuable in 

 the organism. 



Cells which have the ameboid character have usually 

 the power to enclose all manner of foreign particles with 

 which they come into contact. It is by such means that 

 the aquatic amebse secure food. A similar capacity 

 is observed in the case of the leucocytes; they also sub- 

 merge in their own substance various small bodies which, 

 in favorable instances, may be digested and entirely 

 obliterated. This action has a peculiar importance 

 because bacteria are frequently devoured in this way 

 and so we count the leucocytes as defenders of the organ- 

 ism against mischief-making invaders. The process in 

 course of which bacteria are engulfed and destroyed 

 is called phagocytosis which means "scavenging." 

 Around a threatened spot, such as a wound containing 

 dirt, leucocytes in vast numbers are massed in the 

 tissues. The student marvels that they should be 



