AMOEBA 25 



cytoplasm continually drain. As a result the vacuole enlarges. 

 When a certain size is reached, the surrounding endoplasm con- 

 tracts, and the wastes are forced out of the animal into the sur- 

 rounding liquid environment through whatever portion of the 

 ectoplasm happens to be nearest to the contractile vacuole at the 

 time. By this contraction the vacuole is temporarily eliminated, 

 but in a few moments the waste materials will again be present 

 in a sufficient quantity to bring about a reappearance of the vac- 

 uole. A little later, the contractile vacuole having again attained 

 the maximum size, a contraction takes place with a consequent 

 expulsion of the liquid contents as before described. It is probable 

 that this structure also functions in the removal of carbon dioxide. 



4. Reproduction 



When the food supply of an Amoeba is plentiful, so that the 

 animal can ingest, digest, and assimilate food material in such an 

 amount that the katabolic wastes are more than met, growth re- 

 sults. If this process of growth were able to continue indefinitely, 

 an Amoeba of enormous size would soon develop. For some reason, 

 however, Amoebae and other animals are limited, although there 

 is some variation, to a certain maximum size which is character- 

 istic of the particular species. When this limit is reached in an 

 Amoeba, the unicellular animal divides by mitosis into two sepa- 

 rate individuals, each of which when first formed is one-half the 

 size of the parent. This method of reproduction in the unicellular 

 forms is known as binary fission. The two daughter cells begin 

 at once to assimilate food material in sufficient quantities to cause 

 growth, and in a few hours each has attained the normal size. 

 Reproduction by division may again take place, so that within the 

 space of a few hours four independent Amoebae will thus have 

 arisen from the original parent animal. (W. f. 8.) 



If the environmental conditions continue to be favorable, there 

 will be thousands of Amoebae in the course of a few days. The 

 process of growth and reproduction could apparently continue, if 

 the food supply held out and there were no other inhibiting environ- 

 mental influences, until the earth was covered with a mass of 

 amoebic protoplasm. It is a question, however, whether any pro- 

 tozoan cell can continue to divide indefinitely without the occur- 

 rence at certain intervals of some type of reorganization of the 

 nuclear material, such as endomixis in Paramecium. (B. p. 44.) 



