VARIETIES OF THE MALARIAL PARASITE 633 



It will thus be seen that in the human subject the parasite 

 passes through an indefinite number of regularly recurring 

 asexual cycles, with the giving off of collateral sexual cells, and 

 that in the mosquito there is one cycle which may be said to 

 start with the impregnation of the female gamete. 



Varieties of the Malarial Parasite. The view propounded 

 by Laveran was that there is only one species of malarial 

 parasite, which is polymorphous, and presents slight differences 

 in structural character in the different types of fever. It may, 

 however, now be accepted that there are at least three distinct 

 species which infect the human subject. Practically all are 

 agreed as to a division into two groups, one of which embraces 

 the parasites of the milder fevers " winter-spring " fevers of 

 Italian writers there being in this group two distinct species, 

 for the quartan and tertian types respectively ; whilst the other 

 includes the parasites of the severer forms " sestivo-autumnal " 

 fevers, malignant or pernicious fevers of the tropics, or irregu- 

 larly remittent fevers. There is still doubt as to whether there 

 are more than one species in this latter group. Formerly 

 Italian writers distinguished (1) a quotidian; (2) a non-pig- 

 mented quotidian ; and (3) a malignant tertian parasite, though 

 the morphological differences described were slight. Further 

 observations have, however, thrown doubt on this distinction, 

 and the evidence rather goes to show that there is a single 

 species. Opinion also varies as to the cycle of this parasite; 

 according to some observers it is twenty-four hours, according to 

 others forty-eight hours; though there is more evidence in 

 support of the latter view, and the term " malignant tertian " 

 is frequently used. The fever is often of an irregular type and 

 multiple infection is probably common. Although the question 

 cannot be considered as finally settled, we shall speak of three 

 species of human parasites. The zoological position may be 

 shown by the following scheme, generally followed by English 

 writers, the terminology being chiefly that of Grassi and 

 Feletti : - 



Family : H^MAMCEBID^ (Wasielewski). 



Genus I. Hsemamceba. The mature gametes resemble in form the 

 schizonts before segmentation has occurred. 



Species 1. ffcemamceba Danilewski or halter idium. 

 Parasite of pigeons, crows, etc. 



Species 2. ffcemamceba relicta or proteosoma. 

 Parasite of sparrows, larks, etc. 



Species 3. ffcemamceba malarias (Plasmodium malarice}^ 

 parasite of quartan fever_of man, 



