1 92 Arthropods as Hosts of Pathogenic Protozoa 



in these crescents after about half an hour's time. From certain 

 of them there were pushed out long whip-like processes which moved 

 with a very active, lashing movement. The parasite at this stage 

 is known as the ''flagellated body." Others, differing somewhat in 

 details of structure, become rounded but do not give off "flagella." 



The American worker, MacCallum (1897), in studying bird 

 malaria as found in crows, first recognized the true nature of these 

 bodies. He regarded them as sexual forms and believed that the 

 so-called flagella played the part of spermatozoa. Thus, the "flagel- 

 lated body" is in reality a micro gametoblast, producing microgametes, 

 or the male sexual element, while the others constitute the macro- 

 gametes, or female elements. 



It was found that when blood containing these sexual forms was 

 sucked up by an Anopheline mosquito and taken into its stomach, a 

 microgamete penetrated and fertilized a macrogamete in a way 

 analogous to what takes place in the fertilization of the egg in higher 

 forms. The resultant, mobile organism is known as the migratory 

 ookinete. In this stage the parasite bores through the epithelial 

 lining of the "stomach" (mid-intestine) of the mosquito and becomes 

 encysted under the muscle layers. Here the oocyst, as it is now 

 known, matures and breaks up into the body cavity and finally 

 its products come to lie in the salivary glands of the mosquito. Ten 

 to twelve days are required for these changes, after which the mos- 

 quito is infective, capable of introducing the parasite with its saliva, 

 when feeding upon a healthy person. 



Thus the malarial parasite is known to have a double cycle, an 

 alternation of generations, of which the asexual stage is undergone in 

 man, the sexual in certain species of mosquitoes. The mosquito is 

 therefore the definitive host rather than the intermediate, as usually 

 stated. 



The complicated cycle may be made clearer by the diagram of 

 Miss Stryke (1912) which, by means of a double-headed mosquito 

 (fig. 126) endeavors to show how infection takes place through the 

 biting of the human victim, (at A) , in whom asexual multiplication 

 then takes place, and how the sexual stages, taken up at B in the 

 diagram, are passed in the body of the mosquito. 



The experimental proof that mosquitoes of the Anopheline group 

 are necessary agents in the transmission of malaria was afforded in 

 1900 when two English physicians, Drs. Sambon and Low lived for 

 the three most malarial months in the midst of the Roman Campagna, 



