192 MOSQUITOES OF NORTH AMERICA 



nomenon is due the fact that blood examinations made just before a fever- 

 paroxysm may be negative, the parasite not having yet reached the periphery. In 

 the greatest part of the eases there can only be observed annular parasites, gen- 

 erally situated close to the margin of the blood-cell. Rarely there can be found 

 adults and schizonts in process of division. The proportion of parasitized cor- 

 puscles is variable and in acute cases can reach an enormously high number, far 

 beyond what is known to occur in the other two species. 



The gametes have a fonn characteristic of the species. They were the first 

 malarial organisms to be observed. In 1843 Klencke found them but it was 

 Laveran who, in November of 1880, in Algeria, recognized them as the causative 

 agents of malaria, calling them crescentic bodies or half-moon bodies. On 

 account of this very characteristic form of the gametes certain authors have 

 placed the organism in a genus by itself, under the name Laverania, in spite of 

 the fact that this form is transient and changes to a spherical shape when it 

 becomes free in the blood-plasm. 



Ziemann described a form of Plasmodium falciparum which he found in 

 Africa, founding it on the more pronounced roundness of the crescents and the 

 rarity of their appearance in the peripheral blood. This rarity of the crescents 

 is, however, everywhere a frequent occurrence with the pernicious parasite and it 

 is particularly accentuated during the epidemic months. This variety, as also 

 various species indicated by certain investigators, have not been accepted by the 

 majority of good workers. Withal it seems nearly certain that still another 

 species of Plasmodium exists, characterized by annular forms identical with 

 those of P. falciparum, but developing without provoking the acute attacks char- 

 acteristic of this species. 



Plasmodium vivax. — This species is responsible for the benign tertian malaria. 

 It is the best studied of all the species, being the one used by Schaudinn in his 

 famous researches which have thrown so much light on the biology of the 

 malarial parasite. Seen fresh the schizont appears endowed with lively move- 

 ments. Schizogony takes place in periods of 48 hours, each parasite liberating 

 from 12 to 24 merozoits. It is this species which produces the greatest change 

 in the blood-corpuscle. After 16 hours it already presents the characteristic 

 granulations of Schiiffner. The parasite, 24 hours after having invaded the 

 corpuscle, begins to produce an hypertrophy of the blood-cell which becomes more 

 accentuated with the gro^vth of the parasite. Parallel mth this change the 

 granulations of Schiiffner increase in number and the anemia of the corpuscle, 

 wliich begins with its hypertrophy, continues to grow more pronounced. In 

 stained preparations these changes can be very distinctly perceived. The 

 gametes are spherical. The macrogametes when fully developed are about 1 1/2 

 times as large as the normal red blood-cells ; the microgametocytes are very 

 slightly larger than the corpuscles. 



The human organism can be parasitized at the same time by more than one 

 generation of parasites, either of the same species or of different species. These 

 evolve independently of each other and therefore also produce separate fever- 

 paroxysms. The " double benign tertian " is an example which can be cited ; 



