MORPHOLOGY, LIFE CYCLE AND PHYSIOLOGY OF P. FALCIPARUM 
45 
Regarding the question of a capsule in 
the case of gametocytes, our experience has 
been similar to that of Thomson (1932) who 
found that evidence of a periplast was not 
constant. 
Sporogonous Cycle 
Most of the sexual cycle of P. falciparum 
occurs in the definitive host, some member 
of the genus Anopheles. We have noted 
that after a certain number of asexual 
cycles in a falciparum-infected patient, on 
the average about the ninth or tenth day 
after the first detection of parasites, gameto¬ 
cytes commence to appear in the peripheral 
circulation; Thomson (1914) observed a 
ten-day interval. As a rule the female 
forms predominate numerically. The gam- 
etocyte production in falciparum infections 
usually is relatively slow, and by no means 
in every instance are equal or even ap¬ 
preciable densities observed. Nevertheless 
greater densities are commonly encountered 
and they persist for much longer periods 
than with the other plasmodia of man. At 
the time of first appearance of the gameto¬ 
cytes in the peripheral circulation the 
trophozoite count has most often dropped 
to negligible numbers, clinical activity has 
usually ceased and some days later it is not 
uncommon to see the parasites represented 
by gametocytes alone. There would appear 
to be a cause and effect relationship between 
a wave of trophozoite production and the 
succeeding wave of gametocyte production 
since this phenomenon may occur two or 
even three times in close succession during 
a falciparum attack. Thomson and Robert¬ 
son (1935) stated that “experimental evi¬ 
dence, therefore, is lacking which would 
associate the wave of crescent increase—and 
decrease—with any reaction of the host to 
the parasite." Generally speaking, it re¬ 
quires upwards of a week or so after their 
first appearance before the gametocytes ac¬ 
quire sufficient densities or are sufficiently 
mature to infect mosquitoes. The prepara¬ 
tion and examination of stained exflagella¬ 
tion smears will serve as a guide to the 
maturity of the gametocytes. Exflagella¬ 
tion can also be demonstrated in fresh prep¬ 
arations which are kept from drying out. 
When the anopheline mosquito ingests ma¬ 
ture microgametocytes (male) this process 
of exflagellation occurs and from four to 
eight highly motile, thread-like microgam¬ 
etes are released. In the stomach of the 
mosquito the microgamete penetrates the 
macrogamete (female), a fusion of their 
chromatin occurs and the zygote is formed. 
The zygote, which early has a rounded 
shape, becomes elongated and motile; 
known then as an ookinete, it finally pene¬ 
trates the stomach wall and forms a cyst 
(oocyst) subjacent to the outer surface. 
The ookinetes may be found in smears made 
with the stomach contents during the 
twenty-four hours following an infective 
feeding. They may be as long as 20 or more 
microns and as wide as 5 microns but they 
are frequently not symmetrical. They re¬ 
tain some pigment and the chromatin is 
usually centrally located. 
In preparations made after about two 
weeks incubation (at 20-21° C) following 
the infective feeding, well developed, but as 
yet immature, cysts may be seen on the 
stomach wall. After three weeks’ incuba¬ 
tion they may range from 46 to 54 microns 
in diameter and are commonly about 48 
microns. Falciparum cysts are character¬ 
ized by two or three small dark masses of 
pigment which are usually located close to 
one another. Within the cysts the masses 
of sporozoites are arranged in a radiating 
fashion having roughly the appearance of a 
rosette. 
On maturity the oocysts release the sopor- 
zoites into the body cavity of the mosquito. 
These may be found in most of the insect’s 
tissues but of chief importance are those 
which find their way to the salivary glands. 
These glands usually are sporozoite positive 
about three weeks after the infective feed¬ 
ing, if incubated at 20-21° C. It has been 
our experience that mosquitoes with falci¬ 
parum sporozoites are more surely infective 
if they are utilized within the first ten days, 
after the salivary glands become sporozoite¬ 
positive. 
When the proboscis of an infected anoph¬ 
eline penetrates the skin of a person, sali- 
