68 PROTOZOA 



middle. The constriction gradually gets deeper and deeper, cuts through 

 the nucleus, and passes entirely through the body of the animal. The 

 ameba thus becomes divided into two individuals, each half the size of 

 the parent and each with a nucleus half the size of that of the parent. 

 Fission involves a very simple mitosis (Fig. 21 A to D). 



Another form of reproduction is that of sporulation. This occurs 

 only when conditions become unfavorable and is a means of carrying 

 the animal over to a time when normal conditions are again reestablished. 

 It occurs, therefore, when the body of water in which an ameba is living 

 dries up and apparently comes about as a consequence of the increasing 

 salt concentration of the water. It also occurs when the chemical com- 

 position of the water changes in any way so as to be unfavorable or when 

 other environmental conditions threaten the death of the animal. In 

 sporulation, the pseudopodia are first drawn in, and the animal assumes 

 a spherical form; a three-layered cyst is then formed about the surface, 

 which serves for protection (Fig. 21 E). Within the cyst the nucleus 

 undergoes division into a great many fragments, and the cytoplasm 

 becomes divided in such a way that a small amount surrounds each 

 fragment of the nucleus. Thus are formed what are termed pseudopodio- 

 spores (Fig. 21 F). When the pond is again filled with water or when 

 normal conditions are restored, the encysted mass takes up water, bursts 

 its wall, and the liberated pseudopodiospores develop into little amebas. 



