110 
MALARIA 
have included the numbers of gametoeytes 
in the patient’s blood at the time of its in¬ 
gestion by the mosquito. There are evi¬ 
dent examples in these data, as well as in 
the work of malariologists in other parts 
of the world, of individual mosquitoes 
which remained refractory to huge doses of 
infectious gametoeytes. 
The fact that most species of Anopheles 
cannot be bred in small space has pre¬ 
vented their use in breeding experiments. 
The culieine mosquitoes have been more 
suitable for this kind of study. It has 
been possible in mass selection experiments 
with Culex pipiens and Plasmodium cathe- 
merium to lower or raise the percentage 
of susceptible mosquitoes in a stock by 
selecting from uninfected or infected in¬ 
dividuals respectively (Huff 1929). The 
characters responsible for susceptibility 
have been shown to be recessive and to be 
inherited in the 1:3 ratio (Huff 1931). 
Fertilization of the macrogamete and the 
formation of ookinetes apparently take 
place in a species of mosquito regardless 
of whether the latter is susceptible to in¬ 
fection by the parasite. This has been 
shown in my experiments with avian ma¬ 
laria (1927) and by Nicolaew and Yakow- 
lewa (1929) for P. vivax and three species 
of culieine mosquitoes. Ookinete forma¬ 
tion may even occur in vitro. The success 
or failure on the part of the ookinete in 
penetrating the stomach wall is to a greater 
extent than any other incident responsible 
for infection or escape from infection by 
the mosquito. We know (in Culex pipiens) 
that by no means all of the ookinetes suc¬ 
ceed in getting through the stomach wall 
of a susceptible mosquito (Huff 1934). 
That some mechanism is responsible for 
permitting only a certain number to get 
through is shown by the high correlation 
between the numbers of parasites from 
two infectious feedings which, in the first 
brood, get through the wall and which, in 
the second brood, appear to be normal and 
capable of continuing their migration 
through the wall. However, histological 
studies of the gut wall and of the cellular 
elements in the blood meals of susceptible 
and insusceptible individuals have failed 
to show any difference in appearance be¬ 
tween the two individuals (Huff 1934). 
Once the ookinete has penetrated the gut 
wall and become an oocyst it apparently 
runs relatively little chance of failure to 
develop if environmental conditions are 
favorable. A good many malariologists 
feel, however, that stomach infections do 
not constitute as good an index of infec¬ 
tiousness as salivary gland infections. We 
have, of course, evidence in favor of this 
belief. Chitinization of the oocysts may 
result in black spores and such oocysts do 
not usually produce normal sporozoites. 
Barber (1936b) studied the question of de¬ 
generation of sporozoites and was inclined 
to the belief that some of the degeneration 
within the glands began so early that it 
must have been initiated in the oocyst. If 
an inimical, humoral principle exists in the 
blood of the mosquito the sporozoites would 
be exposed directly to it during their pas¬ 
sage from oocyst to salivary gland. Bar¬ 
ber found degeneration to be more fre¬ 
quent in some than in other species of 
Anopheles, which might possibly be inter¬ 
preted as evidence of some inimical prin¬ 
ciple in certain mosquitoes. Boyd (1940c) 
found very good agreement between the 
incidence of stomach and gland infection 
in A. quadrimaculatus infected with P. 
vivax. 
Recent quantitative studies on the 
oocysts of avain malaria in Culex pipiens 
show differences in average size in different 
individuals (Huff 1940). Since these dif¬ 
ferences exist between individuals having 
exactly the same degree of infection and 
since the same mean oocyst size may be 
found in individuals with widely different 
degrees of infection, it would seem to fol¬ 
low that there must be at least two factors 
present in the mosquito affecting the 
growth of the oocyst. One of these regu¬ 
lates the degree of infection and the other 
influences the rate or extent of growth of 
the oocysts. 
Of the environmental factors influencing 
infection in Anopheles, temperature has 
been given the closest study. Grassi 
