THE PROTOZOA 147 



In Plasmodium, moreover, two different forms have been evolved, 

 which have been termed the multiplicative and the propagative forms 

 respectively ; the former serves to multiply in the host, and so bring 

 about a thorough and continuous infection, and the latter serves to 

 infect new hosts. The trophozoites in the blood of the vertebrate 

 by multiple fission produce merozoites or agametes, serving ex- 

 clusively for self-infection. Such part of the life history is termed 

 schizogony, and the trophozoites known as schizonts. The game- 

 tocytes in the stomach of Anopheles produce well-differentiated 

 anisogametes, and these by syngamy form zygotes. The multiple 

 fission of the zygotes leads to the formation of sporozoites, and these 

 are utilised for cross-infection. We distinguish this part of the life 

 cycle as sporogony, and the gametocytes may be termed sporonts. 

 Plasmodium then is also digenetic, exhibiting in a marked form 

 " alternation of generations/' one sexual alternating with an 

 indefinite number of asexual generations. 



