MORPHOLOGY, LIFE CYCLE AND PHYSIOLOGY OF P. VIVAX 
37 
authors that when they are not seen the 
fault is with the staining, but this is cer¬ 
tainly not always true. 
A second change in the host cell is a very 
considerable enlargement which becomes 
apparent even while the parasite is still 
relatively small, and may continue until 
the normal size is doubled (Fig. 3). Along 
with the increase in size and the appear¬ 
ance of the Schiiffner’s dots is a progres¬ 
sive decoloration. These three changes in 
the infected red cell are diagnostic of in¬ 
fections with P. vivax. A fourth peculiar¬ 
ity of this species is the relatively lighter 
shade of brown exhibited by the pigment. 
Sporogonous Cycle 
(In mosquito) 
As in the case of the other species of 
human parasites the final, or definitive, 
host is some species of anopheline mosquito 
In the mosquito only the gametocytes can 
survive the processes of digestion, and 
these begin the changes known as matura¬ 
tion very soon after ingestion. Darling be¬ 
lieved that at least 12 gametocytes per 
cubic millimeter must be present in the 
blood to make the patient infectious for 
mosquitoes, but others have found that in¬ 
fection may take place when there are 
fewer than this (Craig 1928). It is a re¬ 
markable fact that even in insusceptible 
species of mosquitoes maturation of the 
gametocytes can apparently take place, 
and that it will also take place in vitro, as 
for example under the microscope. Indeed 
it was the observation of the phenomenon 
known as “exflagellation,” or microgamete 
formation, which convinced Laveran that 
the pigmented bodies he saw in the blood of 
malarious persons were in reality parasites. 
What the factors are which govern this 
process are still very incompletely known, 
but the work of Marchoux and Chorine 
(1931) on maturation and fertilization in 
Hemoproteus paddae suggests that the pH 
of the medium is the controlling factor, 
and that this in turn is primarily deter¬ 
mined by the amount of carbonic acid pres¬ 
ent. The process in this parasite is stopped 
by a pH of 7.3 or less, but proceeds nor¬ 
mally between a pH of 7.6 and 9.0. 
Maturation of the microgametocyte in¬ 
volves the production of from 4 to 8 mi¬ 
nute flagellated bodies which are then 
known as microgametes (Fig. 13). Raf- 
faele (1939) has described the process in 
some detail, and states that each micro¬ 
gamete consists of two elements, a cyto¬ 
plasmic flagellum and a thin filament con¬ 
taining the fertilizing chromatin. These 
generally adhere for their entire length. 
When maturation is completed, all that re¬ 
mains of the parent microgametocyte is a 
small amount of cytoplasm and chromatin, 
with a mass of pigment. The entire proc¬ 
ess is easily observed under the microscope, 
and will usually take place within a short 
time after the blood is drawn (often in 10 
to 20 minutes). Flagellating microgame- 
/tocytes are recognized by the violent dis¬ 
turbance which the developing gametes 
cause as they endeavor to leave the parent 
cell. 
The changes involved in the maturation 
of the macrogametocyte are no doubt as 
profound as those just outlined, but they 
are less dramatic and less carefully worked 
out. After rupturing the wall of the host 
erythrocyte the gametocyte becomes 
rounded, and it is said to throw off one or 
two tiny masses of protoplasm which are 
generally supposed to represent polar 
bodies. Schaudinn studied the process in 
P. vivax, and also believed that he saw par¬ 
thenogenesis of the macrogametocyte, but 
in this he was quite certainly mistaken. 
Fertilization takes place when matura¬ 
tion is complete, and the zygote (or ooki¬ 
nete, as it is now called) makes its way by 
worm-like movements into the intestinal 
epithelium. Between the epithelium and 
outer wall of the gut it becomes rounded 
and is soon seen to be surrounded by a cyst 
wall. All this may require from 24 to 48 
hours, the time for these and subsequent 
changes depending chiefly on the tempera¬ 
ture. 3 It is said that for P. vivax the op¬ 
timum temperature for development in the 
mosquito is 25° C., although development 
