408 BIOLOGY OF THE PROTOZOA 



of benign tertian malaria of man. He also added the last link to 

 the chain of evidence by watching the penetration of a human blood 

 corpuscle by a sporozoite fresh from a mosquito's proboscis. 



The essential features by which the different types of malaria 

 organisms are distinguished are: (1) Length of time between suc- 

 cessive sporulating periods; (2) relative sizes of parasites and human 

 blood corpuscles; (3) effects of the parasites upon human corpuscles; 

 (4) relative numbers of merozoites formed at sporulation; (5) gen- 

 eral form of the sporulating organisms; (6) distribution of the 

 melanin ; (7) form of gametocytes. 



1. The early history of the trophozoite is much the same in all 

 species. After an initial infection sporozoites enter erythrocytes 

 as minute rounded bodies (Plate I) which soon give rise to ring- 

 shape inclusions (signet-ring stage); these are characteristic of all 

 malaria organisms and, except for size, they are all alike (Plate I, 

 Figs. 1, 7, 13). When fully grown the nucleus divides from three to 

 five times, after which the parasite breaks up into as many merozo- 

 ites as there are nuclei (Plate I, Figs. 4, 10). Upon rupture of the 

 corpuscle the merozoites enter other normal corpuscles and the de- 

 velopmental cycle is repeated. 



Invasion of corpuscles and their destruction thus increases by 

 geometrical progression until a stage is reached and enough toxic 

 substances are freed in the blood to give the first definite clinical 

 symptoms of the disease. Such a period of incubation, i. e., from 

 the time of inoculation to the first clinical symptoms, usually lasts 

 from ten to twelve days. A second convulsion (pyrexial attack) 

 occurs after the merozoites liberated at the time of the first attack 

 have grown to full size and again undergo sporulation. The time 

 required for this growth and reproduction differs with different 

 species and furnishes an important diagnostic character for the 

 identification of species. Thus P. vivax, the cause of so-called 

 benign tertian malaria, since it is rarely fatal, sporulates at forty- 

 eight -hour intervals (every third day); P. malariae, the cause of 

 quartan malaria, on every fourth day or at seventy-two-hour 

 intervals; and P. falciparum, at irregular intervals, from quotidian 

 to tertian. Fever charts of clear cases of tertian, quartan and sub- 

 tertian malaria are thus characteristically different. 



2. While the phases of activity of all species of Plasmodium are 

 alike, there is a distinct difference in size of the parasites as shown 

 by the proportion of the corpuscle that is occupied. P. vivax rarely 

 exceeds three-quarters of the erythrocyte; P. malariae, the largest 

 of the Plasmodium species of man, may occupy as much as nine- 

 tenths; and P. falciparum, the smallest, rarely grows to more than 

 two-thirds the size of the corpuscle. 



3. The effects of the parasites upon the infected corpuscles are 

 likewise different; P. civa.v causes a measurable enlargement 

 (Plate I), while in preparations stained by the Giemsa method 



