MORPHOLOGY, LIFE CYCLE AND PHYSIOLOGY OF P. VIVAX 
39 
certain species) after parasites have largely 
disappeared from the blood. "We may 
therefore believe that the exoerythrocytic 
cycle may be originated from the sporo¬ 
zoites, 5 and that it may continue, presum¬ 
ably at a low level, and possibly play a part 
in the genesis of relapse (Sehuleman 1940). 
But there is also a strong possibility that 
even in those species of avian plasmodia, in 
the cycle of which it occurs most regularly 
and can be most easily demonstrated, the 
occurrence of exoerythrocytic schizogony 
may not be a usual thing but dependent on 
a relative failure of the immune mechan¬ 
ism. As such it may be chiefly significant 
as an expression of what might be called 
the biological potentialities of the parasite, 
and as an indication of the close relation¬ 
ships of the malarial plasmodia to the 
Hemoproteidae. If this is true we may 
have to look still further for stages bridg¬ 
ing the gap between the sporozoite and the 
forms seen in the red cells. 
Physiology 
It is more difficult to give an account of 
the physiology of the malarial parasites 
than to discuss their morphology. This is 
largely because there is still no practicable 
method of cultivation, although the method 
devised by Bass and Johns (1912) makes it 
possible to maintain the parasites for a 
limited time in vitro. The fact that the 
addition of dextrose is required for such 
cultures suggests that the parasites need 
this substance for their vital processes, pre¬ 
sumably as a source of energy. Further 
support for this belief may be had from the 
work of Christophers and Fulton (1939) 
and Fulton (1939), in which it is shown 
that the addition of dextrose to the medium 
causes an increase in the oxygen consump¬ 
tion of P. knowlesi. They also found that 
a number of other sugars (including laevu- 
lose, mannose, maltose, and glycerol) were 
5 Although it is of course necessary to suppose 
that when exoerythrocytic stages occur they must 
arise directly or indirectly from the sporozoites, yet 
it is now quite certain that they may also originate 
from the pigmented forms occurring in the erythro¬ 
cytes (Coulston and Manwell, in press). 
utilized by the parasites, and they remark 
that all of these have a “common chemical 
grouping in their molecule which does not 
include a ketonic or aldehydic group.” 
They found no clear evidence of the phos¬ 
phorylation of dextrose in knowlesi infec¬ 
tions in monkeys. Whether any conclu¬ 
sions applying to the parasites of human 
malaria can be drawn from such experi¬ 
ments is somewhat doubtful. We can be 
quite sure that with the factors which make 
the different species distinctive in morphol¬ 
ogy and host go others, no less real, making 
for similar differences in physiology. An 
example is the demonstration by Coggeshall 
(1940a) that the oxygen consumption of P. 
knowlesi is six times as great as that of P. 
inui. Such characteristics as these are 
probably important in interpreting the 
effects of therapeutic drugs. 
Apparently the globin fraction of the 
hemoglobin molecule is also utilized by the 
parasites, for hemozoin, or malarial pig¬ 
ment, seems to be derived from what re¬ 
mains. This substance seems to be very 
similar to, or perhaps identical with, hae- 
matin (Sinton and Ghosh 1934b; Ghosh and 
Nath 1934). The formula of haematin is 
variously given but CssHgzNiOiFeOH 
may be taken as at least a close approxima¬ 
tion. Since, however, the color and form 
of the pigment produced by the different 
species vary somewhat, it is probable that 
the composition is not always the same. 
The work cited above was done with P. 
knowlesi, a species of monkey plasmodium, 
and is probably more significant than most 
such investigations previously made be¬ 
cause the pigment was extracted directly 
from the parasites rather than from tissues, 
and because it was not previously exposed 
to the action of fixatives. 
There is as yet no agreement as to 
whether there is anything in the nature 
of a toxin produced by any of the spe¬ 
cies of malaria parasites. The pathological 
changes characteristic of malaria can for 
the most part be quite easily explained on 
other grounds. 
The problem of the effect of the parasite 
on the host is to a certain extent tied up 
