190 PATHOGENIC MICROORGANISMS. 



to belong to different varieties, differing in their appearance, 

 their mode of division, and the length of time which they 

 take to go through their whole life-cycle. Three varieties 

 are at present recognized : the tertian, which goes through its 

 life-cycle in man in forty-eight hours ; the quartan, whose 

 life-cycle is three days ; and the aestivo-autumnal, whose life- 

 cycle is indefinite and irregular. 



These different species have constant characteristics, and 

 are not transformed from one into another, though Laveran 

 claims that they are modifications of one and the same 

 species, and are interchangeable. Recent researches have 

 demonstrated that each variety of malaiial parasite is the 

 cause of a particular kind of malarial fever, and by a micro- 

 scopic examination of the blood of a patient one may authori- 

 tatively state the kind of malaria with which that patient is 

 affected. 



In addition to the life-cycle which begins and ends in the 

 human subject, there is another one which only begins in man. 

 For instance, some of the parasite bodies increase in size ; they 

 do not divide, but getting free in the blood-plasma they are 

 found as bodies of characteristic shape, larger than the red 

 blood-corpuscles. These bodies circulate in the blood for a 

 number of days, not giving rise to any phenomena ; and if they 

 remain sterile, they degenerate and disappear. Upon exam- 

 ination it is found that some of those bodies throw off flagella 

 which move with great rapidity among the red blood-cor- 

 puscles ; others do not present this phenomenon. In the 

 aestivo-autumnal parasites, on account of their appearance 

 these bodies have been called crescent bodies. Some persons 

 regard these forms as degenerated forms ; but it has been 

 definitely ascertained that those bodies which degenerate and 

 disappear, when they remain in man are capable in the intes- 

 tines of certain species of mosquitoes of starting a second 

 life-cycle which may be described as follows : When a mos- 

 quito of the Anopheles variety bites a person in whose blood 

 these crescent bodies are present, or their analogous form in 

 the other species of parasites, some of them are taken up 

 with the blood and lodge in the mid-intestine of the mos- 



