142 AN INTRODUCTION TO ZOOLOGY 



same number of separate minute bodies, known as the merozoites. 

 These discrete masses are arranged in a radiating manner around a 

 central mass of residual protoplasm, in which all the melanin has 

 accumulated, and so a very characteristic form, known as the 

 " rosette " stage, is entered upon. The corpuscle then breaks down 

 and the merozoites are set free to attack a fresh set of corpuscles, and 

 the melanin is discharged into the blood. The merozoites behave in 

 precisely the same way as the sporozoites, and a fresh cycle is again 

 gone through. In the recurrent forms of the sickness a day of high 

 fever alternates with non-feverish intervals, and it has been shown 

 that this fever corresponds with the end of the schizogonous cycle 

 and the discharge of the fresh lot of merozoites into the blood. In 

 tertian ague (P. vivax) this occurs every third day, and in quartan 

 ague (P. malaria) every fourth day. The schizogony in P. immacu- 

 latum is not so regular, and the merozoites are discharged constantly 

 into the blood, the result being either a continuous fever or one 

 recurring at quite irregular intervals. 



It will be seen that if this process goes on an increasing number of 

 red corpuscles are infected and destroyed, and after a comparatively 

 few schizogonous cycles an enormous number will have been 

 attacked even from a small original infection. The patient becomes 

 anaemic, the pigment is deposited in the brain capillaries and certain 

 of the viscera, the spleen becomes enlarged and congested, and general 

 cachexia ensues. This may prove fatal, but if it does the parasites 

 are destroyed with their host and not able to reinfect, so that the 

 death of the host is obviously an undesirable event from the point 

 of view of the parasite. We find generally that after a number of 

 schizogonous cycles have been completed the trophozoite does not 

 turn into a schizont. On the other hand, probably as the result 

 of chemical changes in the blood, it does not reach such a large size, 

 but rounds off slightly earlier and turns into a gametocyte. These 

 are of two forms, generally easily distinguishable by the distribution 

 of the melanin within them. The male form, or microgametocyte, 

 has a large nucleus, lightly staining protoplasm, and small pigment 

 granules more or less evenly distributed, while the female form, or 

 macrogametocyte, has a smaller nucleus, more deeply staining 

 cytoplasm, and coarser granules aggregated around the nucleus. 

 In P. immaculatum we find the same differences in nucleus cytoplasm 

 and melanin, but the animals take on a characteristic bent sausage- 

 shape within the corpuscle, being then termed the " crescents," and 

 so are clearly distinguished from trophozoites and schizonts in all 

 stages of growth. No further changes are undergone and no develop- 

 ment is possible on the part of the gametocytes until they are trans- 

 ferred to the next host, a mosquito. 



