THE MORPHOLOGY, LIFE CYCLE AND PHYSI¬ 
OLOGY OF PLASMODIUM VIVAX 
By REGINALD D. MANWELL 
DEPARTMENT OF ZOOLOGY, SYRACUSE UNIVERSITY, SYRACUSE, N. Y. 
Of the three generally recognized species 
of human malaria, Plasmodium vivax is by 
far the most widespread and probably ac¬ 
counts for the majority of cases of malaria 
infection. It occurs over a large part of 
the tropical, subtropical and temperate re¬ 
gions of the earth, and may be regarded as 
perhaps the most important of all the 
causes of disease to which man is heir. Thus 
it is this species which is largely respon¬ 
sible for Osier’s (1923) statement that ma¬ 
laria would probably be voted ‘ ‘ the greatest 
single destroyer of the human race” by 
those who know most about disease, al¬ 
though it is not the most death-dealing of 
the three. It is, however, the cause of an 
infection which is both chronic and dis¬ 
abling, and which at any one time is respon¬ 
sible for millions of cases of illness. Just 
as this is true today, so we can be quite 
sure that it has been since before the dawn 
of recorded history, although as facilities 
for human travel have increased the range 
of malaria has no doubt increased also. 
In view of the importance of malaria it 
is rather surprising that the three varieties 
of the parasite were apparently not clearly 
separated until Golgi did so in 1885. He 
discovered the relationship between the 
symptomatology and stages in the repro¬ 
ductive cycle of the parasites, and soon 
afterward (1886a) found that the parasite 
of tertian malaria differed from that of the 
quartan variety. The species name vivax 
was proposed by Grassi and Feletti in 1890. 
In this paper the morphology and what is 
known of the physiology of the tertian 
malarial parasite will be discussed. 
Morphology, Schizogonous Cycle 
The appearance of the living parasites 
presents nothing of great interest, and it is 
no doubt for this reason that they remained 
so long undiscovered. The younger forms 
appear as small hyaline areas, or simulate 
vacuoles in the erythrocytes which, as 
growth of the parasite takes place, soon 
exhibit granules of yellowish-brown pig¬ 
ment (hemozoin). They also show a very 
active amoeboid movement, and this pecu¬ 
liarity suggested the name vivax. As the 
parasites increase in size however this 
movement becomes less perceptible, until 
finally it almost ceases. 
Malarial parasites are, however, gener¬ 
ally studied in stained preparations, the 
method used being some modification of 
that devised by Romanowsky. The most 
commonly used stains are those of Giemsa 
or Wright. When thin blood smears are 
stained by one or another of these meth¬ 
ods, the youngest parasites appear as mi¬ 
nute masses of rather heavily stained pro¬ 
toplasm (Fig. 1) in which there is rela¬ 
tively little differentiation of chromatin 
and cytoplasm. The former usually ap¬ 
pears reddish or some shade of violet, and 
the latter has a bluish tint. In older para¬ 
sites the differentiation 'is more marked. 
After entering another erythrocyte (ap¬ 
parently in this species the young parasites 
prefer reticulocytes—Eaton 1934; Jacobs- 
thal 1936; Kitchen 1938) the parasite 
soon develops a central vacuole which 
gives it the appearance of a ring, on the 
periphery of which the chromatin appears 
as a small granule, or sometimes as two 
granules. Rings of the latter sort have 
been interpreted as evidence of the occa¬ 
sional occurrence of binary fission (Beach 
1936), not only in this but in other species 
of parasites, both human and avian. These 
ring-shaped parasites may take on a great 
variety of shapes as they increase in size, 
with their pseudopodial processes extended 
here and there throughout the cell (Figs. 
