28 ESSENTIALS OF ZOOLOGY 



formation of two polar bodies. Conjugation takes place, and 

 a zygote is formed which elongates and pierces the stomach 

 epithelium, beneath which it becomes encysted. The en- 

 cysted cell grows and the nucleus followed by the cytoplasm 

 divides, and the resulting cells are termed sporoblasts. The 

 sporoblasts multiply next, producing a large number of minute 

 nuclei which migrate to the surface, become associated with 

 cytoplasmic processes and are freed as sporozoites. These 

 escape into the blood sinuses of the insect and are spread all 

 over the body, finally reaching the salivary glands. In the 

 salivary glands they are very small filiform bodies. This 

 phase of life is that of sporogony. When inoculated to man 

 by the insect they penetrate the red cells and begin the 

 trophozoite phase. 



Should the sex cells formed in man at the end of schizogony 

 not find their way to the mosquito, the female members of the 

 series may develop parthenogenetically that is to say, with- 

 out fertilisation and thus produce a fresh outbreak of the 

 trophozoites, it may be years after the original infection took 

 place. 



Plasmodium is therefore holoparasitic, and alternates 

 between man and the mosquito, feeding and multiplying by 

 schizogony in man and undergoing conjugation and a great 

 multiplication by sporogony in the mosquito. Beginning as 

 a parasite in the ancestors of the nearly allied insects con- 

 cerned, it became adapted to the conditions of existence in 

 the blood of the hosts used by these insects to obtain their 

 food by blood-sucking. It is evident that the disease cannot 

 be spread except by the insect. 



Plasmodium appears to be restricted to gnats and 

 mosquitoes of the dipterous Insect family, Culicidae. The 

 feeding hosts of Plasmodium are man, anthropoid and other 

 monkeys, other mammals, and lizards and snakes. 



Haemoflagellata. Flagellate Protozoa are very common 

 in the alimentary canal of Metazoa, and some of them have 

 become highly specialised and often pathogenic. Trypano- 

 soma is found in the blood of Vertebrates, and in the case of 

 two species is the cause of sleeping sickness in man. The cell 



