VARIETIES OF THE MALARIAL PARASITE 529 



which led Ross to believe that he had before him a stage in the 

 development of the malarial parasite.) A distinct membrane 

 called a sporocyst forms around the zygote, and on subsequent 

 days a great increase in size takes place, and the cysts come to 

 project from the surface of the stomach into the body cavity. 

 The zygote divides into a number of cells called blastophores or 

 sporoblasts, and these again divide and form a large number of 

 filiform cells which have a radiate arrangement; these were 

 called by Ross "germinal rods," but are now usually known as 

 sporozoites or exotospores (in contradistinction to the enhaemospores 

 of the human cycle). The full development within the sporocyst 

 occupies, in the case of proteosoma, about seven days, in the 

 case of the malarial parasites a little longer. When fully 

 developed the cyst measures about 60 /* in diameter, and appears 

 packed with sporozoites. It then bursts, and the latter are set 

 free in the body 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 secretion into the human 

 subject. The sporozoites enter red corpuscles and become 

 amcebulse as above described. Daniels found that in the case 

 of the malignant parasite an interval of twelve days at least 

 intervened between the time of feeding the mosquito and the 

 appearance of the sporozoites 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 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 as proved 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 irregularly 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-pi gmented quotidian ; and (3) a malignant tertian parasite, 

 34 



