Insects and Disease 621 



before Manson wrote, King had already accumulated much evidence, based 

 on epidemiological data, in favor of the theory. A year later (1884), Laveran 

 himself briefly enunciated the same views, on the analogy with Filaria noc- 

 turna. Koch and, later, Bignami and Mendini were also advocates of 

 the theory partly on epidemiological grounds and partly because of a possible 

 analogy with the protozoal parasites of Texas cattle-fever which Smith and 

 Kilborne had shown to be carried by a tick. Hence many observers had 

 independently arrived at the same theory by different routes. . . . 



"To leave these interesting theories and to return to actual observations 

 I should begin by remarking that Manson thought the motile filaments to 

 be of the nature of zoospores that is, motile spores which escape from the 

 gametocytes in the stomach-cavity of the gnat, and then occupy and infest 

 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 amrebulae 

 in the blood-corpuscles of the host become sporocytes, which produce asexual 

 spores (nomospores) ; while other amcebulae become gametocytes, which 

 have no function within the vertebrate host. As soon, however, 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 gametocytes 

 emit a variable number of microgametes the motile filaments which 

 presently escape and wander in search of the female gametocytes. These 

 contain a single macro gamete, or ovum, which is now fertilized by one of the 

 microgametes, 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. . . . Directly MacCallum's discovery 

 was announced Manson saw the important bearing of it on the mosquito. 

 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 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 continu- 

 ous assistance and advice which I received 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 carefully for the 



