6o ZOOLOGY FOR MEDICAL STUDENTS chap. 



within the cavity of the stomach of the mosquito, burrows through 

 its wall and just outside the layer of cells which form the greater part 

 of the thickness of the wall again rounds itself off into a sphere (Fig. 23, N). 

 Its nucleus divides several times in succession and the cytoplasm segments 

 into a number of sporoblasts each containing a nucleus (Fig. 23, 0). 

 Each nucleus now undergoes division a great many times in succession, 

 the small nuclei making their way towards the surface of the sporoblast 

 and a little mass of cytoplasm segregating round each to form a sporozoite 

 (Fig. 23, P). These sporozoites are at first rounded but become spindle- 

 shaped and later much elongated and shaped almost like the microgametes. 

 During the process of sporozoite-formation the spherical mass of sporo- 

 blasts undergoes a great increase in size (Fig. 23, Q) and the number of 

 sporozoites into which, with the exception of a certain amount of residual 

 protoplasm, it is ultimately resolved is very vast. Eventually the delicate 

 membrane enclosing the mass of sporozoites ruptures, the sporozoites 

 are set free in the blood of the mosquito (Fig. 23, R), they bore their 

 way through the cells of the salivary glands (Fig. 23, S) into its duct 

 and when the mosquito next bites are injected with its saliva into the 

 blood of the animal bitten (Fig. 23, T). If this be a human being or other 

 suitable creature the sporozoite attaches itself to a red corpuscle, burrows 

 into it, and becoming an amoebula starts the whole life-cycle afresh 

 (Fig. 23, A). 



Of the various types of malarial fever in man there are three which 

 are particularly well marked and which have been investigated particu- 

 larly completely. These are associated with three different species of 

 parasite — Plasmodium vivax, P. malariae, P. falciparum. 



P. vivax is the parasite of ordinary Tertian fever or Tertian ague 

 as it used to be called. The period occupied by the schizogony is about 

 48 hours so that the fever attack occurs every other day (Fig. 24). A 

 distinctive characteristic is the number of merozoites (15-20) composing 

 the group derived from a single schizont (Fig. 25, B). 



P- malariae is the parasite of Quartan fever or Quartan ague in which 

 the schizogony cycle occupies 72 hours so that two successive fever 

 attacks with the intervening period occupy four days. In it the number 

 of merozoites arising from a single schizont is usually 6-12, and they 

 often are arranged in a very regular rosette (Fig. 25, A). 



P. falciparum is the parasite of " Tropical fever." In this case it is 

 difficult to make certain of the exact period occupied by the process of 

 schizogony owing to the fact that the breaking up of the schizonts takes 

 place usually in the capillary blood-vessels of the brain, spleen and other 



