240 INFECTION AND IMMUNITY 



with blood containing the parasite and the develop- 

 ment of a typical attack of periodic fever as a result 

 of such inoculation. After such an inoculation a 

 period varying from four to twenty-one days elapses 

 before the occurrence of a febrile paroxysm. This is 

 the so-called period of incubation, during which, no 

 doubt, the parasite is undergoing multiplication in 

 the blood of the inoculated individual. The dura- 

 tion of this period depends to some extent upon the 

 quantity of blood used for the inoculation and its 

 richness in parasites. It also depends upon the 

 particular variety of the parasite present, for it has 

 been ascertained that there are at least three distinct 

 varieties of the malarial parasite one which pro- 

 duces the quartan type of fever, in which there is a 

 paroxysm every third day and in which, in experi- 

 mental inoculations made, the period of incubation 

 has varied from eleven to eighteen days ; one in 

 which the paroxysm occurs every second day (ter- 

 tian), in which the period of incubation is from 

 nine to twelve days; and one, denominated the 

 aestivo-autumnal type, in which the period of incuba- 

 tion rarely exceeds five days. 



The parasite associated with each of these types 

 may be recognised by an expert, and there is no 

 longer any doubt that the difference in type is 

 due to the fact that different varieties or " species " 



