PLASMODIA 293 



of the parasite, are extremely active, soon become detached, and 

 move away into the surrounding fluid in search of the female cells. 

 The latter project a small portion of protoplasm on to their surface 

 and into this one of the flagellate sperm cells enters, the protoplasm 

 is instantly retracted, and the fertilized female cell is then spoken 

 of as a zygote or ookinete. In the stomach of a mosquito the 

 zygotes penetrate the stomach wall and settle between the muscle 

 fibers, where on the second day after the ingestion of infected blood 

 they may be seen as small, rounded cells about 8 /* in diameter, 

 containing masses of pigment. Around each zygote a membrane, 

 termed a sporocyst, develops, and as the organisms increase in 

 size they project into the stomach cavity, giving to it an irregular 

 beaded appearance. Meanwhile numerous spherical bodies (sporo- 

 blasts) have been formed within the interior of the zygote and 

 these again divide into a large number of thread-like cells (sporo- 

 zoites). The full development of the sporocyst takes from eight 

 to ten days, after which it bursts, and the liberated sporozoites are 

 carried by the lymphatics to all parts of the body of the mosquito. 

 They settle especially within a large gland which is a combination 

 of a poison and salivary gland and which also is in close connection 

 with the biting apparatus of the mosquito. Hence when an infected 

 mosquito bites a human subject the sporozoites readily pass into 

 the puncture with the gland secretions and the cycle within the 

 human host begins. Since only the female mosquito sucks blood 

 it alone is responsible for the spread of the disease. 



It will be noted that in the mosquito the parasite passes through 

 one cycle only, while in the human host it passes through an indefi- 

 nite number which recur at regular periods. 



So far as is known the parasites of human malaria do not invade 

 any other mammalians nor does any other species of mosquito 

 other than Anopheles harbor them. Anopheles may be distin- 

 guished from the more common Culex by the following character- 

 istics : it appears usually after sunset, while Culex is a day-flying 

 variety ; when at rest its body stands off at an acute angle from 

 the surface of the object on which it is reposing, whereas the body 

 of Culex appears almost parallel with the surface. Many species 



