Reproduction and Life-Cycles 71 



LIFE-CYCLES 



General features 



The simple life-cycles of many species include only an active phase 

 and a cyst. With the cyst apparently eliminated, the "cycle" reaches the 

 limit of simplicity in such types as Entamoeba gingivalis and Pentatricho- 

 monas hom'mis. Other modifications of this basic pattern include: (a) 

 the development of two or more stages in the active phase; (b) the intro- 

 duction of sexual phenomena, which may appear in a sexual phase alter- 

 nating with an asexual phase in the cycle. 



Two or more active stages occur in the life-cycles of many Protozoa. In 

 addition, immature and adult forms of a single organism may be quite 



Fig. 2. 13. AD. Changes preceding division of ilie macronucleus in 

 Eiiplotes; stages in condensation (AC); elongation jnst before division (D); 

 x485 (after Tnrner). E-I. Division of the macroiniclens in Slentor coeruleus; 

 diagrammatic (after \Veisz). 



