192 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 microgametoblast, producing micro gametes, 
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, arc 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, 
