FOOD-TAKING BY AMOEBA 47 



periphery of the cell, is perfectly clear and without solid particles 

 of any kind. It suddenly disappears, the contained fluid being 

 excreted to the outside through the ectoplasmic layer. It 

 shortly reappears as a small clear space which rapidly grows in 

 size until it again disappears by contraction. From its rhythmic 

 dilatation and contraction this organ of the cell is called the 

 contractile vacuole; its pulsations are fairly constant and regu- 

 lar in the same temperature but the rate varies with changes in 

 temperature. This organ was formerly believed to be a beat- 

 ing heart, but is now generally regarded as an excretory or 

 possibly respiratory center. 



Movement --In. the course of its " amoeboid movement' 1 

 Amoeba throws out blunt protoplasmic processes termed 

 psendo podia. The ectoplasm gives way at one point as a 

 result of unknown inner forces ; the endoplasmic granules may 

 be seen streaming toward this point until a blunt finger-formed 

 pseudopodium results. Sometimes the entire mass of the 

 Amoeba flows through this extended portion, and the organism 

 then will have moved a distance equal to its diameter. In the 

 meantime, however, other pseudopodia may be forming and 

 the direction of movement changed. As the cell changes in 

 moving direction the older pseudopodia are withdrawn, and 

 these may be seen as small blunt processes of the cell on the 

 side opposite that in advance (cf. Fig. 10). 



Metabolism- -The constant streaming of protoplasm and the 

 constant formation of pseudopodia require energy. The his- 

 tory of energy transformation resulting in the advance of an 

 Amoeba involves the entire history of metabolism and, if 

 understood, would make the matter of " vital activities' 3 an 

 open secret. Some few points, however, are known. The 

 immediate source of energy for such movements is the com- 

 bustion or oxidation, through the action of enzymes, of parts 

 of the cellular protoplasm, and if no food were taken in, the 

 organism would soon die from loss of its vital parts. This loss, 

 however, is constantly made good by capture and digestion of 

 food, functions involving the primary and fundamental activi- 

 ties of all animals and plants, these functions being the central 



