GENERAL ZOOLOGY 



tivity is exhibited by the amoeba: in the presence of two kinds of prey, 

 equally abundant, the organism ingests the one kind which appears to be most 

 easily digested, and rejects the other. Moreover, the responses are not fixed 

 and mechanical but vary with the physiological state of the amoeba. In the 

 adjustment of reaction to stimulus, and to the state of its physiology, an 

 amoeba behaves in a manner resembling the behavior of multicellular 

 organisms. 



The resemblance between the vital functions in an amoeba and those in a 

 vertebrate may be shown by tracing the history of the ingested food. When 

 a small flagellate, such as Chilomonas, is ingested by Amoeba proteus, the prey 

 continues to move about for several minutes before it is killed by something 

 within the vacuole. Meanwhile, the food vacuole, which at the outset con- 

 tains a relatively large amount of water, shrinks by the difl'usion of excess 

 water into the cytoplasm. The fluid then remaining within the vacuole be- 

 comes alkaline, and in later stages it becomes acid. If the changes in 

 individual vacuoles are followed, the ChUomonas will be seen to disintegrate 

 gradually, until, some 12 or 24 hours later, there remain only certain granules 

 that are apparently indigestible. Fat globules are liberated from the food 

 mass and appear in the vacuolar fluid within 2 or 3 hours, after which they 

 gradually decrease in size and disappear. Starch grains disintegrate into a 

 pasty mass, which disappears as the vacuole slowly decreases in volume. 



The disintegration of other particles and further shrinkage of the vacuole 

 follow, until only a few granules remain. Even these remnants may pass into 

 the endoplasm instead of being egested. Egestion occurs by the discharge 

 of food in various stages of digestion, and of the indigestible residue of food, 

 after all the digestible material has passed into the cytoplasm. Often several 

 vacuoles in late stages coalesce, and the resulting mass comes into contact with 

 the plasmalemma at or near the posterior end of the amoeba. The mass is 

 egested by rupture of this membrane. From observations such as these, it is 

 inferred that fats, carbohydrates, and proteins are digested in the food 

 vacuoles, presumably by specific enzymes, as in the digestive tracts of many- 

 celled animals. It also appears that the products of digestion pass into the 

 endoplasm as they pass into the cells lining the digestive tract of a higher 

 animal. Alternatively, the comparison may be made with the absorption of 

 nutrients from the surrounding lymph by cells in all parts of the vertebrate 

 body. Thus, the digested food is ready to be assimilated. 



Amoebas cannot survive in water from which all dissolved oxygen has been 

 removed, as by boiling; this demonstrates that available oxygen is necessary 

 for the continued existence of these animals. Normally, oxygen diff'uses into the 

 cell from solution in the surrounding medium, just as it enters the cells of a 

 vertebrate from solution in the intercellular lymph. As in the cells of vertebrates, 

 oxygen is essential to cellular metabolism, the series of oxidative reactions which 

 release energy within the cell. The end products of the catabolic process are, 

 as in the vertebrate, carbon dioxide, water, and nitrogenous compounds. These 

 are eliminated from the amoeboid cell by excretion through its surface. Ex- 



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