38 
MALARIA 
may go on slowly even at 15.5 ° C. 4 Growth 
now begins, and the oocyst (Fig. 16) mul¬ 
tiplies its size many times, reaching a diam¬ 
eter of 50 micra, or even more. Micro¬ 
scopically, the chromatin may be seen to 
divide repeatedly until by the time matur¬ 
ity is reached there are almost countless 
fragments. Each of these then appro¬ 
priates, so to speak, a bit of cytoplasm, and 
the whole oocyst breaks up into minute 
elongated forms known as sporozoites. 
These are set .free in the body cavity of the 
mosquito, but eventually some of them 
reach the salivary glands and are then 
ready to infect a new host. The sporo¬ 
zoites resemble the microgametes somewhat, 
but do not possess a separate flagellum, and 
often appear to have a nucleus made up of 
several masses of chromatin (Figs. 14 and 
15). There may be one or many oocysts in 
a single mosquito, depending on the num¬ 
ber of gametocytes in the ingested blood, 
so that the number of sporozoites produced 
may be extremely large. Yet there is no 
reason to think that the malaria infection 
appreciably shortens the life of the in¬ 
fected mosquito. 
The subsequent history of the sporozoite 
is still in doubt, for it is now quite certain 
that it does not enter a red cell immediately 
after reaching a new host. Gordon and 
Lumsden (1939) studied the mechanism of 
biting in Culex mosquitoes, and found that 
blood might be taken either from a pool 
resulting from injury to adjacent capil¬ 
laries, or (more often) directly from a 
pierced capillary. Saliva from the mos¬ 
quito was injected at intervals during the 
feeding, so that contained sporozoites 
would probably be ejected directly into the 
capillaries and carried to remote situations 
very quickly. Boyd and Kitchen (1939) 
confirmed this conclusion in indirect 
fashion when they found that even imme- 
s The time required for the completion of that 
part of the cycle occurring in the mosquito is from 
one to two weeks in the ease of P. vivax. 
4 Stratman-Thomas (1940) states that it will 
not go on at temperatures above 30° C., and that 
24 hours at 37.5° C. will sterilize nearly all Anoph¬ 
eles quadrimaculatus. He also says that in some 
regions oocysts might survive the winter. 
diate excision of the tissues about the area 
bitten failed to prevent the development 
of malaria later. Further, the blood of 
persons bitten by vivax-infected mosquitoes 
has been shown by Raffaele (1937) to be 
non-infeetious to other persons until after 
the fourth day, and he made similar obser¬ 
vations in the case of P. relictum. That the 
sporozoites might go through further de¬ 
velopmental stages was suggested by 
Knowles and Basu (1935), as a consequence 
of the observation that these forms often 
showed evidence of division while in the 
mosquito. Missiroli (1934) showed that 
the sporozoites of P. relictum may pass 
through several divisions after injection 
into the canary, and recently Schuleman 
and Spies (1940) have shown this to be 
true of P. gallinaceum also. Whether still 
another stage intervenes in the cells of the 
reticulo-endothelial system before the ery¬ 
throcytes are invaded is still a matter of 
controversy, and the answer may well be 
different for the different species of plas- 
modia. 
The Possibility of an Exoerythrocytic 
Cycle 
When pigment-free stages were first dis¬ 
covered in certain of the species of avian 
malaria it was thought that similar stages 
might be found to exist in some or possibly 
all of the other species of plasmodia, includ¬ 
ing those of man, and as a result an ener¬ 
getic search has been made by a number of 
investigators. So far, however, exoery¬ 
throcytic stages have been reported only in 
P. vivax (Raffaele 1937) (Figs. 17 and 18) 
and P. falciparum (Casini 1939), while 
their occurrence in these species has not 
been confirmed by others. Raffaele inter¬ 
preted them as forms derived from the 
sporozoites and believed that they gave 
rise to parasites of two series, the one an 
unpigmented series occurring in cells of 
the reticulo-endothelial (lymphoid macro¬ 
phage) system, and the other the familiar 
erythrocytic series. Casini’s observations 
were made on a chronic case. In birds such 
stages are usually seen during the acute 
stage, but may also be seen (especially in 
