PLASMODIUM 



6i 



internal organs and not in the more accessible vessels of the skin. The 

 period is most probably 48 hours (" malignant tertian fever ") but it 

 may be some other period or even irregular. The merozoites are usually 

 8-15 in a group and a marked diagnostic feature is the form of the 

 fully developed gametocytes which until they become free from the 

 corpuscle are sausage-shaped (" crescents," Fig. 25, D). 



One of the unpleasantly interesting characteristics of malaria is its 

 liability to recur in an individual who may have been apparently free 

 from the disease for a prolonged period — several years — and who has 

 not been exposed to any possibility of re-infection. It is clear that this 

 must be due to some of the parasites lurking on within the body after 

 the great majority have died off. It is most probable that during the 

 periods of apparent health a few parasites are all the while going on 



B 



F.G. 25. 



Malarial parasites showing characteristic differences between difierent species. A-C, Schizogony 

 in Plasmodium malarias (A), P. vivax (B), and P. falciparum (C) ; D, gametocyte of P. falciparum 

 enclosed within remains of blood-corpuscle. 



with the normal cycle of schizogony — the total numbers never becoming 

 large enough to cause obvious symptoms — although on the other hand it 

 has been suggested that it is one particular phase of the parasite, namely 

 the macrogametocyte, that is endowed with special powers of resistance 

 and is capable of remaining for long periods dormant, awaiting the onset 

 of favourable conditions, when it bursts into activity, behaves as if it 

 were a schizont, and starts off the infection of large numbers of corpuscles. 



It will be of interest before leaving the subject of the malarial 

 parasites to note the names of the chief workers by whom our present 

 ' knowledge has been built up. The foundation of this modern knowledge 

 may be said to be the discovery by Lankester in 1871 of the first 

 protozoan parasite of a blood-corpuscle — the genus Drepanidium or 

 Lankesterella as it is now called — in the blood of the frog. The special 

 foundations of our knowledge of the malarial parasites of man were 



