VARIETIES OF THE MALARIAL PARASITE 593 



this position. (It was in fact the character of the pigment 

 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, the cysts coming to 

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

 The zygote divides into a number of cells called blastophores or 

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

 tili form cells which have a radiate arrangement; these were 

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

 .<)><> rozoites or exotospores (in contradistinction to the enhaemospores 

 of the human cycle). The full development (sporogony) 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 JJL in dia- 

 meter, arid 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 

 ama-lnihe 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 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, 

 !'>; 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- 



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