?(    3    )
die. It is impossible here to give the reasons why, when the energy with
which a protozoon started life is almost all used up in multiplying by simple
division, sexual forms appear. The answer to such a question would involve
a consideration of the whole problem of the origin of the sexes. It is
sufficient with regard to the malaria parasite to say that after a certain num-
ber of asexual generations have been formed, sexual forms appear, which, if
transferred to the body of another host (the mosquito), can, by a process of fertili-
zation and subsequent growth and division, produce forms which, when
retransferred to the original host, develop into entirely rejuvenated young
parasites with as much energy for asexual reproduction as was possessed by
the original organism.
Parthenojene-
sis.
It is held by some very able observers that the malaria parasite has yet
a third method of reproduction, the process being known as parthenogenesis.
This word is derived from two Greek words, parthenos, a virgin, and genesis,
birth or production, and means in this instance the production of a number
of small parasites from a fully mature female parasite which has not been
fertilized by the male element. It is said that the female forms of the para-
site are capable of continuing to live in the blood long after all other forms
have died off, and that, perhaps months or even years after a patient has
apparently recovered, a special stimulus such as a chill or shock to the
system may rouse these forms to renewed activity, when they reproduce
themselves parthenogenetically and cause the appearance of a fresh crop of
young parasites in the blood, thus occasioning a " relapse."
The parasite's
life history.
The asexual
cycle in man.
With this preliminary explanation we are in a position to describe the
complete life history of the malaria parasite.* It has been stated already
that part of its life is passed in man and the remainder in a particular kind
of mosquito, and for the sake of simplicity we may commence with the
young form of the parasite [schizont]† as it is found inside one of  the red
corpuscles of a man's blood. It will be seen under the microscope as a small
unicellular mass of protoplasm, undergoing amoeboid movement and occupy-
ing but a small portion of the whole corpuscle (Pl. I, 2). It gradually grows
larger, feeding upon the substance of the red blood corpuscle and converting
* A careful study of Plate I will render the following description easily understood.
† The correct names of the different stages of the parasite are inserted  between
square brackets. It is not necessary for the beginner to remember these names.
B 2