10 DEVELOPMENT OF PARASITE [CH. 



to the lumen of the alimentary canal and its ducts. It is 

 probable that the evolution of Hcemoproteus columbce is also 

 restricted to the alimentary canal of its invertebrate host, 

 Lynchia, but in this case the development is rather more 

 complicated than that of the trypanosomes, as it includes a 

 sexual phenomenon. 



The most complex type of evolution is that of the malaria 

 parasite (Plasmodium) in its definitive host, the mosquito. In 

 this case the development commences with the production of 

 sexual forms which only become mature in the stomach of the 

 mosquito, and the result of conjugation is a motile body which 

 bores its way through the gut-wall and comes to rest in or near 

 the outer lining of the gut. It then grows at the expense of 

 the tissues of the mosquito and forms a large spherical cyst 

 in which are developed minute sickle-shaped bodies, the 

 sporozoites. Eventually the latter are set free into the body- 

 cavity of the insect and make their way to the salivary glands, 

 where they come to rest in the salivary ducts. These forms 

 are capable of developing in the blood of the vertebrate host 

 and are inoculated, together with the salivary fluid, when the 

 mosquito feeds. The development of the Haemogregarines 

 and Piroplasmidae in their respective invertebrate hosts seems 

 to resemble that of Plasmodium. Between the simple evolu- 

 tion of T. cazalboui and the highly complicated development 

 of Plasmodium there are numerous gradations, but the 

 majority of protozoal parasites probably pass through a life- 

 cycle comparable with that of Plasmodium. 



The development of the ultra-microscopic virus of yellow 

 fever is peculiar from the fact that it is possible to transmit 

 the infection from one mosquito to another without the media- 

 tion of the vertebrate host. The incubation period of the 

 virus in the mosquito is about 12 days, but after the insect 

 has become infective, if it. is crushed up and fed to another 

 Stegomyia the latter becomes infective. 



The evolution of Filaria in the invertebrate host is very 

 different from that of the Protozoa, as the sexual part of the 

 life-cycle takes place in the vertebrate, in this case the defini- 

 tive host, and the embryonic filariae, contained in the blood, 



