524 



NATURE 



[March 29 1900 



in some of the Coccidiidae. Directly MacCallum's discovery 

 was announced Manson saw the important bearing of it on the 

 mosquito theory. Admitting that the motile filaments them- 

 selves do not infect the gnat, he at once observed that it was 

 probably the function of the zygote to do so — and this time he 

 was perfectly right. 



I must now turn to my own researches. Dr. Manson told me 

 of his theory at the end of 1894, and I then undertook to 

 investigate the subject as far as possible. I began work in 

 Secunderabad, India, in April 1895; and should take the 

 present opportunity for acknowledging the continuous assistance 

 and advice which I received both from Dr. Manson and from 

 Dr. Laveran, and later from the Government of India. Even 

 with the aid of the induction, the task so lightly commenced 

 was, as a matter of fact, one of so arduous a nature that we must 

 attribute its accomplishment largely to good fortune. The 

 method adopted — the only method which could be adopted — was 

 to feed gnats of various species on persons whose blood con- 

 tained the gametocytes, and then to examine the insects care- 

 fully for the parasites which by hypothesis the gametocytes were 

 expected, to develop into. This required not only familiarity 

 with the histology of gnats, but a laborious search for a minute 

 organism throughout the whole tissues of each individual 

 insect examined — a work of at least two or three hours for each 

 gnat. But the actual labour involved was the smallest part of 

 the difficulty. Both the form and appearance of the object 

 which I was in search of, and the species of the gnat in which 

 I might expect to find it, were absolutely unknown quantities. 

 We could make no attempt to predict the appearance which the 

 parasite would assume in the gnat ; while owing to the general 

 distribution of malarial fever in India, the species of insect 

 concerned in the propagation of the disease could scarcely be 

 determined by a comparison of the prevalence of different kinds 

 of gnat at different spots with the prevalence of fever at those 

 spots. In short, I was forced to rely simply on the careful 

 examination of hundreds of gnats, first of one species and then of 

 another, all fed on patients suffering from malarial fever — in the 

 hope of one day finding the clue I was in search of. Needless to 

 say, nothing but the most convincing theory, such as Manson's 

 theory was, would have supported or justified so difficult an 

 enterprise. 



As a matter of fact, for nearly two and a half years my re- 

 sults were almost entirely negative. I could not obtain the 

 correct scientific names of the various species of gnats em- 

 ployed by me in these researches, and consequently used names 

 of my own. Gnats of the genus Culex (which abound almost 

 everywhere in India) I called "grey" and "brindled" mos- 

 quitoes ; and it was these insects which I studied during the 

 period I refer to. At last, the persistently nugatory results 

 which had been obtained with gnats of this genus determined 

 nie to try other methods. I went to a very malarious locality, 

 called the Sigur Ghat, near Ootacamund, and examined the 

 mosquitoes there in the hope of finding within them parasites 

 like those of malaria in man. The results were practically 

 worthless (except that I observed a new kind of mosquito with 

 spotted wings) ; and I saw that I must return to the exact 

 method laid down by Manson. The experiments with the two 

 commonest kinds of Culex were once more repeated — only to 

 ]irove once more negative. The insects, fed mostly on cases 

 containing the crescentic gametocytes of Haemomenas praecox, 

 were examined cell by cell — not even their excrement being 

 neglected. Although they were known to have swallowed 

 living Hcemamoebidse, no living parasites like these could be 

 detected in their tissues — the ingested Hsemamoebidse had in 

 fiict perished in the stomach cavity of the insects. I began to 

 ask whether after all there was not some flaw in Manson's in- 

 duction ; but no — I still felt his conclusion to be an inevitable 

 (me. And it was at this very moment that good fortune gave 

 me what I was in search of. 



In a collecting bottle full of larvae brought by a native from an 

 unknown source I found a number of newly-hatched mosquitoes 

 like those first observed by me in the Sigur Ghat — namely, 

 mosquitoes with spotted wings and boat-shaped eggs. Eight of 

 these were fed on a patient whose blood contained crescentic 

 gametocytes. Unfortunately I dissected six of them either pre- 

 maturely or otherwise unsatisfactorily. The seventh was 

 examined, on August 20, cell by cell ; the tissues of the stomach 

 (which was now empty owing to the meal of malarial blood 

 taken by the insect four days previously being digested) were 



NO. 1587, VOL. 6t] 



reserved to the last. On turning to this organ I was struck by 

 observing, scattered on its outer surface, certain oval or round 

 cells of about two to three times the diameter of a red blood- 

 corpuscle — cells which I had never before seen in any of the 

 hundreds of mosquitoes examined by me. My surprise was 

 complete when I next detected within each of these cells a few 

 granules of the characteristic coal-black melaniti of malarial 

 /fz'er— a substance quite unlike anything usually found in mos- 

 quitoes. Next day the last of the remaining spotted-winged 

 mosquitoes was dissected. It contained precisely similar cells, 

 each of which possessed the same melanin ; only the cells 

 in the second mosquito were somewhat larger than those in 

 the first. 



These fortunate observations practically solved the malaria 

 problem. As a matter of fact, the cells were the zygotes of the 

 parasite of remittent fever growing in the tissues of the gnat ; and 

 the gnat with spotted wings and boat-shaped eggs in which I 

 had found them belonged (as I subsequently ascertained) to 

 the gtrwis Anopheles. Of course it was impossible absolutely to 

 prove at the time, on the strength of these two observations 

 alone, that the cells found by me in the gnats were indeed 

 derived from Httmamcebidae sucked up by the insects in the 

 blood of the patients on whom they had been fed — this proof 

 was obtained by subsequent investigations of mine ; but, guided 

 by the presence of the typical and almost unique melanin in the 

 cells, and by numerous other circumstances, I myself had no 

 doubt of the fact. The clue was obtained ; it was necessary on ly 

 to follow it up— an easy matter. 



The preparations of the stomachs of the two Anopheles were 

 sealed, and were afterwards examined by Drs. Smyth, Manson, 

 Thin and Bland-Sutton ; and an account of the work and of 

 the observations of these gentlemen was published a little later. 

 Unfortunately, my labours now met with a serious interruption ; 

 but not before I had succeeded again in finding the zygotes in 

 two other mosquitoes — one, another species of Anopheles, also 

 bred from the larva, and also fed on a case containing crescentic 

 gametocytes; the other, a "grey mosquito" {Culex pipiens 

 type), which had been caught feeding on a case of tertian fever, 

 and which I now think had become previously infected from a 

 bird with Haemamoeba relicta. 



Early in 1898, mainly through the influence of Dr. Manson, 

 Sir H. W. Bliss and the United Planters' Association of South- 

 ern India, I was placed by the Government of India on special 

 duty in Calcutta to continue my investigations. Unable to work 

 with human malaria — chiefly on account of the plague-scare in 

 Calcutta — I turned my attention to the Hsemamcebidae of birds. 

 Birds have at least two species of HaemamoebidDe. I sub- 

 jected a number of birds containing one or the other of these 

 parasites to the bites of various species of mosquitoes. The 

 result was a repetition of that previously obtained with the 

 human parasites. Pigmented cells precisely similar to those 

 seen in the Anopheles were found to appear in gnats of the 

 species called Ctilex fatigans, Wiedemann, when these had been 

 fed on sparrows and larks containing Haemamoeba relicta. On 

 the other hand, these cells were never found in insects of the 

 same species when fed on healthy birds or on birds containing 

 the other parasite, called Haemamoeba danilewskii. 



It will be evident that this fact was the crucial test both as 

 regards the parasitic nature of these cells and as regards their 

 development from the hsemocytozoa of the birds ; and it was 

 not accepted by me without very close and laborious experiment. 

 The actual results obtained were as follows : — 



Out of 245 Culex fatigans fed on birds containing H. relicta, 

 178, or 72 per cent., contained " pigmented cells." But, out of 

 41 Culex fatigans fed on a man containing crescentic gameto- 

 cytes, 5 on a man containing immature tertian parasites, 154 on 

 birds containing H. danilewskii, 25 on healthy sparrows, and 

 24 on birds with immature H. relicta — or a total of 249 insects, 

 all carefully examined — not one contained a single "pigmented 

 cell." 



Another experiment was as follows : — Three sparrows, one 

 containing no parasites, another containing a moderate number 

 of H. relicta, and the third containing numerous H. relicta, 

 were placed in separate cages within three separate mosquito- 

 curtains. A number of Culex fatigans, all bred simultaneously 

 from larvse in the same breeding bottle, were now liberated on 

 the same evening partly within the first mosquito-netting, 

 partly within the second, and partly within the third. Next 

 morning many of these gnats were found to have fed themselves 



