PROTOZOA 



31 



between individuals of a pure line, but it is not permanent, though it 

 may be inherited for several generations before it disappears. It would 

 seem that, apart from the occasional appearance of mutations, the 

 permanent varieties in a species are produced only by syngamy. 



Here may be mentioned the union of individuals by fusion of 

 their cytoplasm, the nuclei remaining distinct, which is practised by 

 the Mycetozoa (Fig. 73 F) and in some other cases. This process, 

 which is not syngamy, is known as plastogamy, and its product as a 

 plas7nodium. 



Fig. 28. A diagram of the nuclear changes in Paramecium aurelia during 

 endomixis. From Robertson, after Jennings. The white circles represent 

 degenerating nuclei. Fissions take place between D and E, and between 

 H and I. 



The life of a protozoon passes in the course of generations through 

 a cycle in which individuals of different kinds succeed one another. 

 The life cycles of various protozoa differ greatly, being related to the 

 vicissitudes of the environment of the species and to the need for 

 distribution as well as to the recurrence at intervals of conjugation, 

 but it is possible to formulate a type of which all of them may be 

 regarded as variants. After a period of "vegetative" existence and 

 increase by asexual reproduction, during which the individuals are 

 known as agamonts, there appears a generation known as gamonts 

 because they Tprod\icQ gametes ^ the latter unite in pairs, and the zygote 

 or sporont gives rise to a generation of sporozoites which, becoming 

 agamonts, repeat the asexual part of the cycle. The table on p. 32 

 shows this typical life history. 



In comparing this table with the actual course of the cycle in any 

 species, the student should remember: (i) that in each part of the 



