ONE-CELLED ANIMALS 



37 



remain perfectly sterile that is, no living animal nor plant 

 will ever appear in it. 



Reproduction in Protozoa. All life comes from life. Every 

 living creature is the offspring of some other living creature. 

 This is just as true for the Protozoa as for the higher animals, 

 but their method of reproduction is usually much more simple. 

 In many cases the Protozoan animal simply divides into two 

 more or less similar smaller animals which grow until they 

 attain a certain size and then divide again, and the process is 

 continued for generation after 

 generation. This is called repro- 

 duction by simple division or fis- 

 sion. 



Protozoa that thus live and re- 

 produce by simple division have 

 been called immortal, and it would 

 seem that under natural condi- 

 tions such animals never die, for 

 as soon as they reach a certain 

 size they divide and form two new 

 individuals and as this process is 

 continued for generation after gen- 

 eration there would seem to be no 

 death of the individual. Careful 

 studies have shown, however, that this process of simple divi- 

 sion cannot continue indefinitely unless there is introduced 

 into the cycle from time to time the extraordinary process 

 known as fertilization by which the mature or old individuals 

 are rejuvenated. In many of the Protozoa this process of 

 fertilization is accomplished by the conjugation of two similar 

 individuals in which two animals come together and undergo 

 complete or temporary fusion. Such a conjugation is followed 

 by renewed activity, the process of division going on more 

 rapidly than before. 



Many Protozoa instead of dividing directly into two parts 

 go through a process called spore formation. The animal 

 becomes encysted in a firm little sac or cyst in which it remains 

 for some time. Then it divides into many small bodies, called 



FIG. 12. Division of 

 Amoeba. (Greatly magnified; 

 after Schultze.) 



