POLYMORPHISM AND LIFE-CYCLES 181 



number of amoebulse which develop into the adult in the usual way 

 (Fig. 80, MQ). 



Arcella thus furnishes a surprising example of diversity both in 

 the courses taken by the development and in the methods of 

 syngamy. We may now consider some further complications of 

 the life-cycle, which in other Protozoa takes usually a more definite 

 and stereotyped course, less liable to the variations in one and the 

 same species seen in Arcella. 



One of the commonest complications introduced into the life- 

 cycles of Protozoa is the differentiation of sexual and non-sexual 

 cycles. In the account given above of the life-cycle of Arcella, it 

 has been seen that an adult may produce amcebulse which as 

 agametes can grow up directly into the adult form without syngamy, 

 or which as gametes copulate before developing further. The 

 adult Arcellce, however, do not, so far as is known, exhibit any 

 differentiation in relation to these developmental differences, the 

 form that produces gametes being perfectly similar to that which 

 produces agametes. But in other cases there may be two distinct 

 forms of the adult individuals : the one, known as the sporont or 

 gamont, which gives rise to gametes ; the other, termed the schizont or 

 agamont, which produces agametes.* In this way an alternation of 

 generations is brought about in which the life-cycle as a whole 

 becomes a combination of two distinct types of developmental cycle 

 one known as schizogony, in which no sexual processes occur ; 

 the other as sporogony, in which at one stage gamete-formation is 

 followed by syngamy. 



An example of alternation of generations in a free-living form is 

 seen in the life-cycle of Trichosph cerium (Fig. 81), as described by 

 Schaudinn (146). The adult phase is a relatively large amoeboid 

 form, approximately spherical in contour, and having the body 

 surrounded by a gelatinous envelope in which at intervals there are 

 apertures through which the lobose pseudopodia are extruded ; the 



* The word " sporont " was a modification suggested by Biitschli for the terra 

 " sporadin," originally coined by Aime Schneider to denote the adult spore- 

 forming phase in the cephaline Gregarines (p. 339), and to distinguish it from the 

 earlier phase which still bears the epimerite, known as a cephalont (" cephalin," 

 Schneider). Since the production of resistant spores in Gregarines and allied 

 orders, such as the Coccidia, is accompanied by sexual phenomena, the word 

 " sporont " has undergone both an extension and a change in its original meaning, 

 and has come to be used to denote a gamete-producing form. In his memoir on 

 Trichosph cerium, Schaudinn used the word " sporont " in this sense, and coined 

 the term schizont to denote the agamete-producing form, and further coined the 

 words " schizogony " and " sporogony " to denote the non-sexual and sexual 

 cycles respectively. Since the word " sporont " in the secondary meaning thereby 

 given to it has reference solety to the occurrence of syngamy and not to the forma- 

 tion of resistant spores, and since these two processes are not always, though 

 frequently, combined in the same series of generations, it would perhaps be better 

 to replace the terms " schizont " and " sporont " by " agamont " and "gamont " 

 respectively, were it not that this leads to the substitution of the extremely cacopho- 

 nous words "agamogony " and "gamogony " for "schizogony" and "sporogony." 



