GENERAL ZOOLOGY 



Fig. 8.22. Paramecium: binary fission. The 

 macronucleus simply elongates and constricts, 

 whereas the micronucleus divides by a 

 mitotic process. 



though encystment has been described. Perhaps it occurs more frequently in 

 nature, since it is difficult to understand how any protozoan can be so uni- 

 versally distributed in fresh water without undergoing occasional encystment 

 to survive periods of drought. Paramecium, however, does not appear to 

 encyst upon aquatic vegetation; it is rarely, if ever, obtained by placing such 

 vegetation in sterile water. In the laboratory the life cycle is an endless active 

 phase with frequent reproduction by transverse binary fission, which is an 

 asexual process. Periodic phases of nuclear reorganization, termed endomixis, 

 also occur. Reproduction by conjugation, or temporary union of individuals 

 with exchange of nuclear material, may also be observed. Some strains of 

 Paramecium, however, appear capable of maintaining themselves indefinitely, 

 by fission and endomixis, without conjugation. 



In the course of binary fission, by which reproduction is accomplished, the 

 macronucleus divides amitotically by elongation and constriction; the micro- 

 nucleus undergoes a kind of mitosis (Fig. 8.22). As division of the two nuclei 

 nears completion, the cell body becomes constricted and finally separates into 

 two daughter cells of equal size. Meanwhile, one new contractile vacuole 

 has been formed for each daughter, and a new gullet has arisen in each from 

 the oral region of the parent cell. After separation the daughter cells usually 



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