PROTOZOAN PARASITES 



295 



definitive host, which may be different from the intermediary host 

 in which the asexual reproduction is found. 



Parthenogenesis and Etheogenesis. — Reproduction is said to take 

 place from a female type of parasite (parthenogenesis) or more 

 rarely from a male type of parasite (etheogenesis), without any 

 completion of the sexual process (Fig. 47). 



An example of parthenogenesis is found in the malarial parasite, in which 

 the female gametocyte is capable of resisting both the action of the chemicals 

 of the body and drugs, and is therefore capable of lying dormant for some 

 time. When given an opportunity for development, its nucleus and proto- 

 plasm divide into two portions, one of which degenerates and disappears, 

 while the other forms merozoites and starts the cycle of schizogony anew. 

 Another is described by Prowazek in Herpetomonas musccs-domesticcB , but 

 neither example has been definitely proved ; and, indeed, of late grave doubts 

 have arisen as to its truth. Nevertheless, we feel that it is necessary to keep 

 some remarks upon the subject, as we feel that the last word has not yetj|,been 

 said, as we have seen once bodies very like those described by Schaudinn, 



Fig, 



47. — Parthenogenesis of Plasmodium 

 vivax Grassi and Feletti. 

 (After Schaudinn.) 

 I, Macrogametocyte ; 2, division of the 

 nucleus; 3-6, formation of merozoites from 

 one portion of the nucleus, and separation 

 of the other portion with the hsemozoin. 



Fig. 46.- — Sporogony of 

 Plasmodium vivax Grassi 

 AND Feletti : Zygosis. 

 (After Schaudinn.) 



I, Zygosis of one micro- 

 gamete with the macro- 

 gamete ; 2, ookinete and 

 degenerate microgametes . 



Etheogenesis, a term introduced by Prowazek, is the much rarer 

 change in the male parasite whereby asexual reproduction begins 

 again. He described it in Herpetomonas musccB-domesticce, but Flu 

 says that an error was made, and that the objects believed to be 

 male elements were really stages in the life-cycle of a Microsporidian 

 • — Octosporea musccB- domestic ce. 



Pathogenicity.— The pathogenicity of the Protozoa is of the 

 highest importance, as they are the causes of a large number of 

 endemic and epidemic diseases in man and animals, as will be 

 described in the following pages. They produce toxins; but these 

 have not been well studied, with the exception of the Sarcosporidio- 

 toxin, which is only toxic for rabbits and not for other animals. 

 The toxins produced by the malarial parasites have been referred 

 to on p. 203. 



