l68 FLEAS, FLUKES AND CUCKOOS 



Stomach. Afterwards the zygote becomes surrounded by a membrane 

 partly secreted by the tissue of the host and partly by the parasite itself. 

 The Plasmodium, which at this stage is kno\vn as an oocyst, continues to 

 grow and tlie nucleus multiplies by schizogony thus giving rise to 

 numerous minute daughter nuclei. Then the cytoplasm begins to 

 break up and form finger-like processes into each of which a nucleus 

 passes. In this way numerous spindle-shaped sporozoites are formed 

 which eventually break a\vay from their point of attachment and remain 

 as a tangled mass within each oocyst. Sometimes as many as 30 or 40 

 such oocysts, each containing up to 10,000 sporozoites, are found 

 beading the surface of a gnat's stomach — all in different stages of 

 development, \\lien ripe the oocyst bursts, liberating the mass of sporo- 

 zoites in the body caN-ity of the mosquito. These pointed, spindle- 

 shaped cells move about by waves of peristaltic contraction and by a 

 gliding motion, by means of which they insinuate themselves into every 

 organ of the mosquito's body. Large numbers reach the sahvar)- glands 

 and pass up the duct \\-ith the saliva. During tlie insect's next blood 

 meal they are injected into the blood stream of the bird and the asexual 

 cycle begins once again. 



In recent vears some extremelv interestins; ^vork has been carried 

 out by James and Tate in England, and by Huff in the U.S.A., using 

 the fowl malaria parasite {Plasmodium gallinaceum) . They have demon- 

 strated that the initial asexual cycle in the bird is passed in the white 

 blood corpuscles and in the endothelial cells of the spleen, heart and 

 brain. This exoer\"throcytic development is follo^\"ed by invasion of the 

 red blood corpuscles in which the parasite continues to multiply by 

 schizogony. These discoveries paved the way to similar discoveries 

 made by Shortt relating to the human malarial parasite and have 

 proved ver\- valuable for studying problems of relapse and treatment 

 of malaria in man. 



The time required for the completion of the sexual cycle in the 

 mosquito varies \Nith the temperature. Under certain conditions it may 

 take only five days for a female mosquito to become infective, but in 

 other cases sporozoites only appear in the sahva after two months have 

 elapsed. 



Some species of bird Plasmodium, of which P. relictum is a good 

 example, are easily transmitted to different kinds of birds, but others 

 show more or less ^^-ell-marked host specificity'. P. gallinareum, which is 

 a parasite of the fowl, will not develop naturally in any other bird, 



