, _a-g_, Cycle of forty-eight hour development, the
period of chill coinciding with the appearance of _f_ and _g_ in the
blood. The organisms _g_, which result from segmentation, attack other
corpuscles and a new cycle begins. _h_, The male form or
microgametocyte, with the protruding and actively moving spermatozoa,
one of which is shown free. _i_ and _j_ are the macrogametes or female
forms. _k_ shows one of these in the act of being fertilized by the
entering spermatozooen. The differentiation into male and female forms
takes place in the blood, the further development of the sexual cycle
within the mosquito.]

If a mosquito of the species _anopheles_ bites the affected
person, it obtains a large amount of blood which contains many
parasites. Within the mosquito the parasite undergoes a further
development into male and female sexual forms, which may also form in
the blood, termed respectively _microgametocyte_ and _macrogamete_.
From the microgametocyte small flagellate bodies, the male sexual
elements _microgametes_ or _spermatozoa_, develop and fertilize the
_macrogametes_; after fertilization this develops into a large body,
the _ooecyst_ which is attached to the wall of the stomach of the
mosquito. Within the ooecyst, innumerable small bodies, the
sporozoites, develop, make their way into the salivary glands and are
injected into the individual who becomes the prey of the mosquito,
and again the cycle of development begins. The presence of the
parasite within the mosquito does not constitute a disease. So far as
can be determined, life goes on in the usual way, and its duration in
the insect is not shortened.

The nature of the parasite which produces yellow fever is unknown, for
it belongs to the filterable viruses; the infectious material,
however, has been shown by inoculation to exist in the blood, and the
disease is transmitted by a mosquito of another species, the
_stegomyia_. The development cycle within this takes a period of
twelve days, which time must elapse after the mosquito has bitten
before it can transmit the disease. Here again the mutual
interdependence of knowledge is shown. Nothing could have seemed less
useful than the study of mosquitoes, the differentiation of the
different species, their mode of life, etc., and yet without this
knowledge discoveries so beneficial and of such far-reaching
importance to the whole human race as that of the cause and mode of
transmission of malaria and yel