184 THE PROTOZOA 



and grows up either into a sporont or into a schizont again. The 

 sporont possesses only a single large nucleus, the primary nucleus 

 originally present in the amcebula, and a great number of chromidia. 

 When the sporont enters upon the reproductive phase, the primary 

 nucleus degenerates, and an immense number of secondary nuclei 

 are formed from the chromidia. Then the protoplasmic body divides 

 up to form as many cells as there are secondary nuclei. The cells 

 thus produced are the gametocytes, each of which divides by mitosis 

 to form four small cells, the gametes, which acquire flagella, swim 

 off, and copulate with gametes produced from another sporont ; 

 there appear, however, to be no differences exhibited by the gametes 

 of opposite sexes. The zygote forms a shell and grows into a 

 sporont. Since the zygote is very much smaller than the amcebula 

 produced by schizogony, the shell formed by it is also smaller. This 

 shell is later the initial chamber of the polythalamous adult, and 

 thus leads to a dimorphism in the adult shells, so-called ' ' micro - 

 sphseric ' and " megalosphaeric ' forms (p. 235) a dimorphism 

 related, in this case, not to the manner hi which the adult individuals 

 reproduce themselves, but to the manner in which they have been 

 reproduced. 



In free-living forms the alternation of generations is related to 

 external conditions of the environment, as, for example, seasonal 

 changes ; the sexual generation may appear in the autumn, while 

 the non-sexual generations are found in the spring and summer. 

 In parasitic forms, on the other hand, alternation of generations is 

 of common occurrence in relation to a change of hosts. Thus, in 

 the life-cycle of the Coccidia (Fig. 152), described above, the multi- 

 plicative phases reproduce non-sexually by schizogony/ as the so- 

 called " endogenous cycle " ; the propagative phases are preceded 

 by gamete-formation, leading to spore-formation, the so-called 

 " exogenous cycle." In Haemosporidia, such as the malarial parasites, 

 for example (Fig. 156), the alternation of generations is related to 

 an alternation of hosts ; the non-sexual, schizogonous generations 

 take their course in the blood of the vertebrate host, in which the 

 gamonts are produced, but do not develop further unless taken up 

 by the invertebrate host, in which alone gametes are formed and 

 sporogony takes place. 



The phrase " alternation of generations " must not be construed 

 into meaning that the sexual and non-sexual generations succeed 

 each other in a regular alternation. On the contrary, such regular 

 alternation, if it occurs at all, is rare, and as a rule a single sexual 

 generation is followed by several, or it may be by an immense 

 number, of non-sexual generations before the sexual cycle recurs. 

 The malarial parasite can multiply non-sexually in the blood for 

 many years without dying out ; and if propagated artificially f rori one 



