1900.] on Malaria and Mosquitoes. 301 



ami then occupy and infect the tissues of the insect. In this he was 

 proved, two years later, to have been wrong. The motile filaments 

 are not spores, but microgametes — that is, bodies of the nature of 

 spermatozoa. I have said that some of the anioabulae in the blood- 

 corpuscles of the host become sporocytes, which produce asexual 

 spores (nomospores) ; while other amcebulse become gametocytes, 

 which have no function within the vertebrate host. As soon, how- 

 ever, as these gametocytes are ingested by a suctorial insect they 

 commence their proper functions. As their name indicates, they are 

 sexual cells — male and female. About fifteen minutes after ingestion 

 (in some species), the male gametocyte emits a variable number of 

 microgametes — {he motile filaments — which presently escape and 

 wander in search of the female gametocytes. These contain a single 

 macrogamete or ovum, which is now fertilised by one of the micro- 

 gametes, and becomes a zygote. We owe this beautiful discovery to 

 the direct observation of MacCallum (1897), confirmed by Koch 

 and Marchoux, and indirectly by Bignami. Metchnikoff, Simond, 

 Schaudinn and Siedlecki have also demonstrated what are practically 

 sexual elements in some of the Coccidiidse. Directly MacCallum's 

 discovery was announced Manson saw the important bearing of it on 

 the mosquito theory. Admitting that the motile filaments themselves 

 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 1884, 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 ac- 

 knowledging the continuous assistance which I received both from 

 Dr. Manson and from Dr. Laveran, and later from the Govern- 

 ment 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 adojjted — the only method which could be 

 adopted — was to feed gnats of various species on persons whose blood 

 contained the gametocytes, and then to examine the insects carefully 

 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 



