MALARIAL FEVER. 523 



■cavity. A large number settle within the large veneno-salivary gland of 

 •the insect, and are thus in a position to be injected along with its secre- 

 tion into the human subject. Daniels found that in the case of the 

 malignant patasite an interval of twelve days at least intervened between 

 the time of feeding the mosquito and the appearance of the filiform 

 spores in the gland. 



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 

 gametocyte — a cycle, moreover, in which the parasite reaches the most 

 advanced stage of its development. 



Varieties of the Malarial Parasite. — The view propounded by La- 

 -veran was that there is only one species of malarial parasite, which is poly- 

 morphous, and presents slight differences in structural character in the 

 different types of fever. It may, however, now be accepted as proved 

 that there are at least three distinct species which infect the human sub- 

 ject. This is shown by their distinct morphological characters, by differ- 

 -ences in the length of their cycle of development, and also, to a certain 

 extent, by their pathogenic effects. 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 ItaUan writers, — there being 

 here 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 distin- 

 guished (i) a quotidian, (2) a non-pigmented quotidian, and (3) a 

 malignant tertian parasite, though the morphological differences de- 

 scribed we'rfe- slight. Further observations have, however, thrown doubt 

 on this distinction, and the evidence rather goes to show that there is a 

 ■single species, which probably has a cycle of forty-eight hours, though 

 variations may occur; multiple infection is moreover common, and thus 

 a quotidian or irregular type may occur. MansOn, for example, consid- 

 ers that if there exists a true quotidian fever it must be of the rarest 

 occurrence. Although the question cannot be considered as finally set- 

 tled, we shall accordingly 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 : — 



