THE PROTOZOA 145 



and they move about actively when they become free. When one 

 approaches a macrogamete this cell puts out a small protuberance, 

 the " cone of reception," generally from that part of the circum- 

 ference nearest the nucleus, and to this the microgamete adheres. 

 Gradually the microgamete is absorbed, and its male pro-nucleus 

 travels to a position near the female pro-nucleus. Shortly after 

 this fertilisation is completed by the fusion of the two pro-nuclei 

 to form the synkaryon or fertilisation nucleus. A single cell thus 

 results, which is the zygote, but, as it enters into a motile stage in 

 Plasmodium, it is frequently spoken of as the ookinete. This leaves 

 the alimentary canal of the mosquito by boring through the epithelial 

 lining of the wall of the stomach and embedding itself in the sub- 

 mucosa. Here it secretes around itself a fairly soft cyst, the 

 oocyst. and becomes truly parasitic on the mosquito, feeding on its 

 juices and increasing in size until it forms a knob as large as a grain 

 of millet projecting on the outside of the stomach. The formation 

 of the zygote in Plasmodium differs considerably from that in 

 Monocystis, for it results from the union of two gametes very 

 different in appearance ; a male or microgamete, and a female or 

 macrogamete. Thus it constitutes an example of Anisogamy or 

 the fusion of dissimilar gametes, Anisogametes. 



After a growth period the ookinete enters on a phase of asexual 

 reproduction, leading to the formation of sporozoites. Its nucleus 

 and cytoplasm divide up, producing a number of masses known as 

 the sporoblasts, not completely isolated from one another, but united 

 by fine protoplasmic strands and leaving a small portion of residual 

 protoplasm. The sporoblast in turn undergoes multiple fission ; a 

 very large number of daughter nuclei are formed, which take up a 

 peripheral position, and a corresponding number of delicate pro- 

 cesses are given, into each of which a nucleus migrates. The residual 

 protoplasm breaks down in each case, setting free an enormous 

 number of very minute sporozoites. Finally, the oocysts burst, 

 releasing the sporozoites into the blood stream of the mosquito, 

 where they are carried about until they come to the salivary glands. 

 They leave the blood at this point and accumulate in vast numbers 

 in the gland. 



Now when the mosquito bores into a small vessel to suck the 

 blood it injects a drop of saliva through the proboscis into the 

 wound, and this has the effect of preventing the blood from coagu- 

 lating. It is very obvious that if the Anopheles happens to be 

 infected a great many sporozoites get poured into the wound with 

 the saliva, and so the man becomes infected in this manner. Thus 

 the whole cycle is ready to start all over again. An interesting point 

 in this connection is that the asexual production of the sporozoites 



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