MORPHOLOGY, LIFE CYCLE AND PHYSIOLOGY OF P. VIVAX 
31 
2-6). Eventually, however, if the parasite 
is destined to reproduce asexually, the chro¬ 
matin divides twice and finally into many 
small masses, and in the latter condition 
is generally known as a segmenter. Fig. 7 
shows an immature schizont, and Fig. 8 one 
almost completely mature. The young 
parasites produced as a result of the mul¬ 
tiple division process are known as mero¬ 
zoites. The number produced by a single 
segmenter (Fig. 9) varies between 12 and 
24, with an average of about 16. It is prob¬ 
able, however, that both the range and the 
average may vary, for it has been shown 
that different races of plasmodia exist 
within a species, and also (in avian mala¬ 
rias) that larger numbers of merozoites 
are produced by segmenters early in the 
course of an infection than are produced 
later (James and Ciuca 1938; Boyd 1940a). 
The schizogonous cycle requires 48 hours. 
Sporogonous Cycle 
(In man) 
Most of the merozoites enter other red 
cells and repeat the process of growth and 
reproduction. There are some, however, 
which grow but exhibit no evidence of 
nuclear division, and these are the sexual 
stages, or gametocytes. The forms can be 
distinguished from reproducing forms of 
similar size by the presence of a single mass 
of chromatin which, in the female or macro- 
gametocyte, is compact and usually situa¬ 
ted near the margin of the parasite. In the 
male, or microgametocyte, it is more dif¬ 
fuse and stains less intensely. The cyto¬ 
plasm of the female forms takes a deeper 
blue color, and contains larger and more 
numerous granules of pigment than that of 
the microgametocyte. In both male and 
female cells the pigment granules are scat¬ 
tered, in contrast to the asexual parasites. 
The cytoplasm of the microgametocyte usu¬ 
ally takes a relatively pale bluish tint, but 
it may also appear a marked purple or 
lilac (Craig 1928, quoting St. John). 
When the cytoplasm stains in this way it is 
quite possibly because of greater maturity. 
Gametocytes of both sexes are shown in 
Figs. 11 and 12. They are unable to de¬ 
velop further unless ingested by a female 
mosquito belonging to the genus Anopheles. 
The distinguishing characteristics of the 
species, and also of the different stages are 
tabulated in Table I. 
TABLE I 
Distinguishing Characteristics of 
Plasmodium vivax 
(in Romano wsky-stained preparations) 
Characters per¬ 
taining to 
Species character 
Rings* 
Usually larger than falcipa¬ 
rum; smaller than malariae; 
stages other than rings also 
present. 
Trophozoites 
Markedly amoeboid in appear¬ 
ance; strap-like or band¬ 
shaped forms seldom seen; 
present in peripheral blood. 
Schizont 
Larger than in other species 
and present in peripheral 
blood. 
Number of mero¬ 
12—24; average about 16. 
zoites per seg¬ 
menter 
Gametocytes: 
Microgametocyte 
Rounded; larger than in ma- 
lariae. Smaller than female 
form; nucleus diffuse and 
often marginal; cytoplasm 
takes pale-bluish stain (or 
may appear lilac or pur¬ 
ple) ; pigment scattered, 
granules appear as short 
rods. 
Maerogametocyte 
Rounded; larger than in ma- 
lariae. Larger than male 
form; nucleus more compact 
and often marginal; cyto¬ 
plasm stains a deeper blue; 
pigment scattered and more 
abundant than in micro- 
gametoeyte, granules appear 
as longer rods. 
Effect on host cell 
Markedly enlarged; partially 
decolorized (especially in 
unstained preparations) ; 
Schiiffner’s dots usually 
present. 
* Identification cannot usually be safely at¬ 
tempted from the rings alone. 
At this point the question as to what 
determines the fate of a given merozoite 
may well be asked. Why do some give rise 
