32 COLLEGE ZOOLOGY 



way of this organ. Oxygen dissolved in water is taken in through 

 the surface of the body. This gas is necessary for the life of the 

 animal ; if replaced by hydrogen, movements cease after twenty- 

 four hours; if air is then introduced, movements begin again; 

 if not, death ensues. 



GROWTH. - - If food is plentiful, more substance is added to 

 the living protoplasm of the A meba than is used up in its various 

 physical activities. The result is an increase in the volume of 

 the animal. This is growth, and, as in all other living organisms, 

 growth by the addition of new particles among the preexisting 

 particles, i.e. growth by intussusception. 



Reproduction. - - There is, however, a limit with regard to the 

 size that may be attained by Ameba proteus, as it rarely exceeds 

 .25 mm. ( T ^- - inch) in diameter. When this limit is reached the 

 animal divides into two parts. Why should there be such a 

 limit? The following explanation is given by Herbert Spencer 

 and others. The volume of an organism varies as the cube of 

 its diameter, the surface as the square. Thus, as an animal 

 grows, the ratio between surface and volume decreases; and, 

 since Ameba takes in food, gives off waste material, and carries 

 on respiration through its surface, the activities of the cell must 

 decrease with increase in size until further growth is impossible. 

 The solution of the problem is the division of the animal into 

 two, whereby the ratio of surface to volume is increased. Re- 

 production by binary division, therefore, takes place when 

 growth is no longer possible. It is supposed that this division 

 is inaugurated through some unknown change in the relations 

 between the nucleus and cytoplasm. There are at least two 

 kinds of reproduction in Ameba proteus, but neither has ever 

 been satisfactorily worked out in detail. They are (i) binary 

 division and (2) sporulation. 



(i) During binary division (Fig. n) the nucleus divides by a 

 primitive sort of mitosis. Then the animal elongates, a constric- 

 tion appears near the center, and division into two daughter cells 

 finally takes place. 



