CHAPTER VII 

 THE REPRODUCTION OF THE PROTOZOA 



THE methods by which reproduction is effected amongst the 

 Protozoa vary greatly in matters of detail, as will be seen ; but the 

 obvious diversity in method throws into greater relief the under- 

 lying unity in principle. In Protozoa, as in Protista generally, 

 reproduction takes place always by means of some form of fission 

 that is to say, division or cleavage of the body into two or more 

 parts, which are set free as the daughter-individuals. An essential 

 part of the process is the partition amongst the daughter-individuals 

 of at least some part of the chromatin-substance possessed by the 

 parent. Hence fission of the cell-body as a whole is always pre- 

 ceded by division of the nucleus ; and if chromidia are present, they 

 also are divided amongst the products of the fission of the body. 

 On the other hand, division of the nucleus is not necessarily 

 followed at once by division of the body. 



Considering the methods by which fission is effected from a 

 general standpoint, we may distinguish three chief types of repro- 

 duction, each of which may show subordinate variations : 



1. Division of the nucleus, or, if there are two differentiated 

 nuclei, division of each of them, is followed by division of the body ; 

 this is the commonest and most typical mode of reproduction, 

 tnown as simple or binary fission. 



2. Division of the nucleus or of each of two differentiated nuclei 

 is not followed immediately by corresponding divisions of the body, 

 but may be repeated several times, and so give rise to a multi- 

 nucleate condition of the body, which may be 



(a) Temporary, and soon followed by cleavage of the body into 

 as many daughter-individuals as there are nuclei or pairs of dif- 

 ferentiated nuclei; this method is known as multiple fission (Fig. 

 127) ; or it may be 



(b) Permanent, giving rise to a multinucleate body which is termed 

 a plasmodium. Then division of the body may take place at any 

 time by cleavage of the body into two or more multinucleate 

 parts ; this process is known as plasmotomy. Ultimately, however, 

 in all cases a plasmodium breaks up by multiple fission into uni- 



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