REPRODUCTION 



169 



Asexual Reproduction: Fission. — Fission is a common reproductive 

 method among the protozoa, and occurs less commonly among the 

 metazoa. The essentials of fission are that the parent cell or the parent 

 body (if a metazoon) be divided into approximately equal parts, each of 

 which grows and regenerates the misvsing parts and thus comes to resemble 

 the parent. The parent disappears as an individual and two new indi- 

 viduals take its place. The plane of fission may be longitudinal or 



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Fig. 138. — Successive stages in the fission of a single Paramecium. (Courtesy of Ralph 

 Wichterman and General Biological Supply House.) 



transverse. Transverse fission, the more common type, is illustrated in 

 Fig. 138, which shows, step by step, the division of Paramecium cauda- 

 tum. Structures which extend across the plane of fission are divided, 

 and the missing portion regenerated. Other structures go with that 

 portion in which they are located before fission, and corresponding 

 structures arise anew in the other portion. Thus in a Paramecium with 

 two contractile vacuoles, one placed anteriorly, the other posteriorly, 

 one vacuole goes to each new individual and a second vacuole arises anew 

 in each, usually before division is completed, as in the figure. In forms 

 which have both macro- and micronuclei, both nuclei elongaste and finally 



