i8o COMPARATIVE ANATOMY 



aestivo-autumnal or tropical malaria. The two first are com- 

 paratively mild in their effects, and are known as " benign " 

 intermittent fevers; the third is a .pernicious and generally 

 continuous fever. In each case the acute feverish symptoms 

 coincide with the termination of the schizogonous cycle of the 

 life history of the parasite, when the merozoites are set free 

 in the blood and are attacking numbers of previously healthy 

 corpuscles. The three different kinds of fever are attributable 

 to three different species of parasite belonging to the genus 

 Plasmodium (also named Hsemamceba). The parasite causing 

 tertian ague is Plasmodium vivax : in this species the 

 schizogonous cycle is completed in forty-eight hours, and the 

 fever recurs every other day. The parasite of quartan ague 

 is Plasmodium malarias : in this species the schizogony takes 

 seventy-two hours, and consequently the fever recurs once 

 every three days. In Plasmodium immaculatum, the parasite 

 of pernicious tropical malaria, the period occupied by 

 schizogony is irregular, and as merozoites are constantly being 

 discharged into the blood the fever is either continuous or 

 recurs at irregular intervals. 



From what precedes it is obvious that the number of infected 

 red blood corpuscles in a malarial patient increases in 

 geometrical progression, and more rapidly in the case of 

 tropical than in the benign forms of fever. After a certain 

 number of schizogonous cycles a vast number of red blood 

 corpuscles are destroyed, the patient becomes anaemic ; 

 melanin pigment is deposited in the spleen, liver, kidneys, and 

 capillaries of the brain ; the spleen becomes swollen and con- 

 gested, and a condition of general cachexia is produced which 

 may terminate fatally. If the process of asexual multiplication 

 were to continue indefinitely, a fatal result would be certain, 

 and this would be as destructive to the parasite as to its host. 

 But, probably as the result of changes in the blood consequent 

 on the destruction of a large number of red corpuscles, in due 

 time the perpetuation of the parasite is provided for by the 

 production of a number of sexual forms called gametocytes. 

 These sexual forms are of two kinds ; male forms, known as 

 microgametocytes, and female forms known as macrogameto- 

 cytes. Both are developed from the younger forms of the 

 trophozoites, and are generally, though not always, very clearly 

 distinguishable from one another by the distribution of the 



