R E S E A R C H E S O N M A L A R I A
39
pared for the microscope), and was known to occur in from about
10
to 30
minutes after the blood is drawn from the patient; but it was now necessary
to follow it in the mosquito. The insects were killed from one minute to sev-
eral hours after feeding, the stomach being then extracted and its contents ex-
amined in the fresh state. I was obliged to invent the technique for myself; and
my first successful dissection was made on the 13th May. Within a few weeks
I made a fairly complete study of the subject and ascertained an encouraging
fact. In vitro, doubtless owing to the unnatural conditions, only about five per
cent of the crescents give issue to the motile filaments; but I now found that
in the mosquito’s stomach something like sixty per cent of them do so. It was
also noted that the preliminary stages of the process, namely the swelling up
and rounding of the crescents, were much more constantly seen in the blood
ingested by the insect than in vitro. This was of importance because it showed
at least that the insect’s stomach is a more favourable locus for the process than
an ordinary specimen of blood is. I observed also that a considerable percent-
age of the crescents (about one-third) never produced motile filaments at all,*
even after the lapse of several hours and within the insects; and I noticed that
those which refused to emit them had a slightly different appearance to the
others. At the time I thought that they were parasites which had been killed
in some manner during ingestion; but when I repeated the experiments next
year I saw cause to doubt this, and felt some difficulty in explaining why all
the crescents did not emit filaments as they should have done according to
Manson’s hypothesis regarding their nature.
A description of these first results was written in June, but was not published
until the end of the year
24
.
10. Difficulty of the task. New
methods devised. The fact, then, was established
that the gametocytes are not immediately killed in the mosquito’s stomach
(as might well have happened), but indeed emit their motile filaments more
readily there than in vitro. It was necessary now to seek the destination of the
latter in the insect’s tissues; and here the true magnitude of the task to which
I had set myself became manifest. Manson had been able to follow the migra-
tions of the filaria embryos with comparative ease because they are large or-
ganisms readily distinguishable from the fluids or tissues which surround
them. But the motile filaments are exceedingly delicate bodies, the move-
ments of which are very difficult to follow even with the highest powers of a
* It was easy to distinguish those which had produced the filaments by their collapsed
condition afterwards.
40
1 9 0 2 R . R O SS
good microscope and in the clear spaces of an ordinary preparation of blood.
But the blood in a mosquito’s stomach shortly becomes a thick grumous mass
in which it is impossible even to see the filaments unless they are in active
movement. Moreover, even this assistance was denied me; for I speedily ascer-
tained that within a few minutes of their escape they seemed to lose their
movements. At least they constantly disappeared as if by magic; and in spite
of all artifices I failed to ascertain what had become of them.* In fact to trace
them further, to follow the migrations of these well-nigh invisible bodies
through the masses of cells of which so large an animal as a mosquito is com-
posed was indeed an impossible task with the means which I possessed.
Hence, though Manson then and later constantly advised me in his letters to
adhere to the plan of following the motile filaments, I determined to abandon
the quest and to emply other methods; and he himself failed to obtain any
results with the insects which I sent to him from time to time. It was most
fortunate that I came to this decision so early, because events have proved that
the motile filaments migrate nowhere, and do not enter the mosquito’s tissues
at all.
The first method which I now adopted and which ultimately led to success
was the following. By hypothesis, the motile filaments after reaching the par-
ticular cells of the mosquito for which we thought they were destined, should
grow in them into some unknown but larger form. It was impossible to pre-
dict what this form would be. Manson conjectured that it would very likely
be some intracellular form similar to the intracorpuscular stages of the organ-
ism in the human blood - fixed perhaps in the stomach cells or blood cells of
the insect. Personally, however, while I thought this view possible, I had no
full faith in it. It seemed to me that the mosquito stage of the parasite might
be anything, so long as it was of a protozoal character.
The protean changes of many of the parasitic worms warned us that nature
was capable of ordering any extraordinary transformations in the interest of
parasites; and as no case was yet known of a protozoon capable of wandering
from one species of host to another, I had no guide as to what might happen
with the organisms which I was studyin g, and conjectured that for all we
could say, the motile filaments might develop into almost any form - amoe-
boid, coccidiform, gregarinoid, or even infusorial, small or large. What was
nearly certain, however, was that they were likely to grow in size after a few
* It might now be possible, though still difficult, to follow them by staining them either
in dehaemoglobinized blood or in section; but the Romanowsky reaction was not known
to me at that time.