282 GENERAL ZOOLOGY 



When the advancing pseudopodia come in contact with a 

 minute unicellular organism, the object may be enveloped; if 

 so, it gradually sinks into the cytoplasm, the amoeba mean- 

 time continuing its rolling locomotion; if the object is useless 

 for food, the amoeba simply rolls over it, leaving it behind. 

 Food is taken in with a small amount of water which forms 

 a surrounding bubble or vacuole. This food-vacuole is carried 

 about in the endoplasm and slowly disappears in the process 

 of digestion, which goes on quite as effectively in the amoeba 

 as in any of the higher animals. The indigestible particles of 

 the food are left behind as the amoeba rolls along. 



All the other phases of metabolism go on as in many-cell 

 animals, except that in the amoeba everything must be done 

 in the one cell. The oxygen which the animal needs probably 

 comes in through the general surface of the ectoplasm by 

 osmosis. Oxidation and the release of energy take place, as 

 is manifested in locomotion. The waste of the body is prob- 

 ably partly expelled by the contractile vacuole. This organ 

 may be observed in that portion of the ectoplasm which is 

 behind in locomotion. The products of metabolism when 

 brought into that region form a sphere of liquid which, as 

 it increases in size, is carried back from the region of the 

 nucleus to the point where it contracts and bursts on the 

 surface. 



Reproduction in Amoeba is accomplished by the nucleus 

 and the protoplasm dividing into equal portions, the two new 

 cells separating immediately and growing to the size of the 

 original one. This kind of reproduction is considered by 

 many zoologists to be a phase of growth. The cell, by feed- 

 ing, becomes so large that the surface through which food and 

 oxygen pass is not great enough to supply the more rapidly 

 increasing volume of cytoplasm. When division takes place, 

 masses are produced which have enough surface to supply the 

 interior with material for growth and other forms of energy. 



