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 amoebulae 
in the blood-corpuscles of the host become sporocytes, which produce asexual 
spores (nomospores); while other amoebulae 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 
