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323 
In some species their general form and appearance 
is very like that of the sporocytes before the 
spores are differentiated ; in another, however, they 
possess a special crescentic form. They continue to 
circulate in the blood for some days, or even weeks 
(according to the species), without change; but when 
they are drawn into the alimentary canal of certain suc- 
torial insects, they undergo further development. In a 
few minutes after finding themselves in their new position, 
they break from the enclosing corpuscle by a kind of 
expansion, swell up slightly, and then commence their 
sexual functions. The male gametocytes emit a variable 
number of microgametes, which escape into the serum of 
the ingested blood, leaving behind a residuum consisting 
chiefly of pigment—as in the case of the sporocytes. The 
individual microgametes are delicate but very active 
filaments, consisting chiefly of chromatin, and sometimes 
seen to have a slight swelling at one point of their length 
—in the middle, when free, according to my observations. 
After escape from the male gametocyte, the microgametes 
seek the female gametocytes, which consist each of a 
single motionless macrogamete. One microgamete now 
enters bodily into a macrogamete and fertilises it, pro- 
ducing a zygote. 
The further history of the zygote has been traced, as 
regards three species of this group of parasites, in certain 
kinds of mosquitoes. After being fertilised, it acquires 
the power of escaping the phagocytes of the ingested 
blood which surround it, of working its way through the 
mass of blood, of passing through the thickness of the 
stomach (middle intestine) of the mosquito, and of 
affixing itself to the outer surface of the organ. Here 
the zygotes are first found as oval cells about 8-10 » in 
diameter. They still contain the granules of pigment 
which the macrogametes possessed before fertilisation. 
Growing rapidly, they soon acquire a capsule, and begin 
to protrude into the body-cavity of the mosquito. From 
an early period the nucleus divides into a number of 
portions—zygotomeres—each containing a fragment of 
chromatin. As growth advances, the zygotomeres be- 
come spherical blastophores, bearing each a large 
number of delicate, filamentous zygotoblasts on their 
external surface—each zygotoblast being affixed to the 
surface of the blastophore by one extremity. As maturity 
is approached the zygote, though still attached to the 
outer wall of the insect’s stomach, protrudes freely into 
the body-cavity; its pigment tends to disappear ; and 
lastly, the blastophores disappear, leaving the capsule 
packed with thousands of zygotoblasts. Maturity is 
reached in from one to three weeks, according to the 
external temperature, when the zygote reaches a size of 
60 » or more. The capsule then ruptures, pouring the 
zygotoblasts into the insect’s blood. 
The zygotoblasts are now seen to be delicate flagel- 
lula or mastigopods, about 12-16 » in length, with the 
chromatin and one or two unstained areas in the middle, 
and two opposite tapering flagella. I have not, however, 
observed any notable movement in them, probably on 
account of the necessary dissecting medium. After 
being discharged into the insect’s blood, these bodies 
are carried away by the current into all parts of the 
tissues, and finally effect an entry into the large grape- 
like cells of the salivary gland—especially the cells of 
the short middle lobe—where they accumulate in very 
large numbers. From the cells of this gland they pass 
into the duct which runs to the extremity of the middle 
stylet, the lingula, and thence escape during haustellation 
into the blood of a new vertebrate host. Here, it must 
be supposed, the flagellulae attack the corpuscles and 
become the intra-corpuscular amoebulee with which we 
started. 
Four points require notice. (1) There are reasons 
for supposing that the gametocytes are or may be pro- 
duced by the conjugation of two or more amcebulz in 
NO.20558, VOL. 6c] 
NATURE 
one corpuscle. (2) The gametocytes of several species 
show, after escape from the corpuscle, one or more 
minute spherical bodies attached to their margin ; which 
I assume to represent polar bodies. (3) The young 
zygote of at least one species (of crows) possesses, 
shortly after fertilisation, somewhat active powers of 
locomotion. (4) The mature zygotes of at least two 
species often contain large, brown, thick-shelled, cylin- 
drical bodies, the nature of which has not yet been 
elucidated, and which may be parasitic fungi of the 
mosquito. 
A brief history of how these facts came to be ascer- 
tained may be of interest. Laveran discovered the 
human parasites in 1880; and Danilewsky those of 
birds, some years later. Golgi established the law of 
endogenous reproduction by means of spores in 1885 ; 
and noted the differences in the various human species. 
Later, several Italian writers observed the distinction 
between the sporocytes and the gametocytes, but failed 
to understand the nature and object of these latter 
forms. The escape of the microgametes can be wit- 
nessed zz vifro, and a dispute now arose as to the 
meaning of these bodies. Antolisei, Grassi, Bignami, 
Labbé and others held that they are products of death 
and degeneration zz vztvo. On the other hand, Laveran,. 
Danilewsky and Mannaberg supposed them to represent 
the highest development of the organisms, while the 
last writer thought that they were meant for an ex- 
ogenous saprophytic existence—without, however, sug- 
gesting the mode of their escape. In 1894 Manson 
concluded that the gametocytes are intended to continue 
the species outside the vertebrate host; and that they 
escape into the stomach of a suctorial insect, and then 
give rise to flagellule—as he considered the micro- 
gametes to be—which in turn develop in the tissues of 
the insect. He founded these views chiefly on the fact 
that the microgametes escape from the gametocyte only 
after abstraction of the blood from the vertebrate host. 
Laveran had already surmised that the mosquito is the 
alternative host of the human parasites ; and Manson 
now claimed the mosquito as the suctorial insect 
referred to. 
Early in 1895 I attacked the subject experimentally in 
India, on the lines laid down by Manson. Owing to the 
difficulty of the investigation and to the use of wrong 
species of mosquitoes, I failed for more than two years 
in reaching positive results. In August 1897, however, 
on employing two species of Anopheles fed on patients 
containing the crescentic gametocytes, I found the zygotes 
in various stages of growth attached to the wall of the 
insect’s stomach. My work was now interrupted ; but 
next year I succeeded in following out the life-history of 
the zygotes of one of the parasites of birds in their de- 
velopment in Culex pipiens. The zygotoblasts were 
found in the salivary glands of the mosquitoes ; and 
lastly, in July 1898, I succeeded in infecting a large 
number of healthy birds by the bites of infected insects. 
Meanwhile (1898) MacCallum had discovered the true 
nature of the microgametes by actually witnessing the 
sexual act 2 vitro, while Metchnikoff, and Simond had 
found microgamates also in Cocctdium oviforme and 
C. salimandrae,; so that the exact relationship between 
the gametocytes in the blood of the vertebrate (inter- 
mediary) hosts and the zygotes found by me in the 
mosquitoes (definitive hosts) became quite evident. 
My results were published by Manson in August 1898, 
and were confirmed by Daniels, of the Malaria Commis- 
sion of the Royal Society, in December. It was now easy 
to extend my observations to other species of the group, 
a work, however, which I was unable to undertake. In 
November and December 1898, Grassi, Bignami and 
Bastianelli cultivated two of the human parasites in a 
third species of Anopheles, 4. claviger, Fabr., and suc- 
ceeded in infecting several healthy persons. Shortly 
