46 ORGANISMS OF ONE CELL 



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 and 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 beating 

 heart but is now generally regarded as an excretory or possibly 

 respiratory center. 



Movement. In the course of its "amoeboid movement" 

 Amoeba throws out blunt protoplasmic processes termed 

 pseudopodia. 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 moving direction the older pseudopodia are withdrawn and 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 requires energy. The his- 

 tory of energy transformation resulting in the advance of an 

 Amoeba involves the entire history of metabolism and if we 

 but knew it, the matter of " vital activities" would be an open 

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

 source of energy is the combustion 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 activities of all animals and plants and the 

 central factors in the development of the multifold structures 

 in the living world. 



Amoeba proteus captures food and draws it into the body 

 protoplasm through the agency of the pseudopodia, or rather, 



