36 



COLLEGE ZOOLOGY 



Secretion 



Very little is known about secretion in 

 the amoeba. Undoubtedly digestive fluids are 

 secreted into the food vacuoles. Other sub- 

 stances of use in the life processes of the 

 animal may also be secreted. 



Excretion 



The amoeba probably gets rid of most of 

 its excretory matter, including urea and 

 carbon dioxide, through the general surface 

 of the body. The contractile vacuole may 

 serve in part for excretion, but its primar}' 

 function is to regulate the v^^ater content of 

 the cell body. Water enters the body with 

 the food; it is a by-product of oxidation; and 

 it also passes into the cell through the gen- 

 eral surface. The contractile vacuole is 

 formed by the fusion of minute droplets of 

 liquid. Its "wall" is not usually permanent; 

 it is a condensation membrane that disap- 

 pears at each contraction. It forms in various 

 parts of the body, often near the nucleus, 

 and is carried toward the posterior end. The 

 discharge of the contractile vacuole to the 

 outside seems to take place through the up- 

 per surface and for that reason cannot ordi- 

 narily be seen. 



Respiration 



The amoeba requires oxygen for metabo- 

 lism and must get rid of carbon dioxide. This 

 interchange corresponds to the internal 

 respiration of cells in higher animals. That 

 oxygen is necessary for the life of the amoeba 

 can be proved by replacing it with hydrogen; 

 movements cease after 24 hours; if air is 

 then introduced, movement begins again; if 

 not, death ensues. Oxygen dissolved in water 

 is taken in, and carbon dioxide passes out 

 through the surface of the amoeba. The 

 contractile vacuole may take part in carry- 

 ing carbon dioxide to the outside. 



Reproduction 



Ordinarily the amoeba builds up proto- 

 plasm more rapidly than it breaks it down; 



and when full size is reached, it reproduces 

 by the simple process of dividing into two 

 amoebas. This method of reproduction is 

 called binary fission (Fig. 15). The nucleus 

 divides by mitosis (Fig. 15); the prophase 

 lasts 10 minutes, the metaphase probably 

 less than 5 minutes, the anaphase about 10 

 minutes, and the telophase about 8 minutes. 

 The nuclear membrane disappears during 

 the metaphase. The body of the amoeba, at 

 the time of division, becomes spherical and 

 covered with small pseudopodia; it elongates 

 and separates into two during the telophase 

 stage in mitosis. The time required for the 

 entire process depends on the temperature; 

 at 24° C. it takes about 33 minutes, and 

 under laboratory conditions, the amoeba di- 

 vides every few days. 



Development in the amoeba is simply a 

 matter of growth; the rate of growth is 

 rapid just after division, and then gradually 

 decreases until the size for division is once 

 again reached, which takes on the average 

 about three days. Potentially, the amoeba is 

 "immortal," for if it reproduces by fission, 

 there is no death from old age. If death oc- 

 curs, it results only from an accident. 



Behavior 



The activities of the amoeba involving 

 changes in shape, formation of pseudopodia, 

 locomotion, capture of food, etc., constitute 

 its behavior. These activities are due largely 

 to changes in the animal's environment and 

 possibly in part to internal changes such as 

 "hunger." The environmental change is 

 called a stimulus, and the animal's reaction, 

 a response. The amoeba responds to a num- 

 ber of types of stimuli, including those due 

 to changes in contact, light, temperature, 

 chemicals, and electricity. Movement toward 

 a stimulus is called a positive reaction and 

 away from a stimulus, a negative reaction. 



Contact 



The amoeba when touched with a small 

 rod will cease locomotion for a time and 



