Page:Tropical Diseases.djvu/984

926 is applied to the young schizont or sporont which, is absorbing nourishment direct from the cell in which it is lodged. Parthenogenesis is not recognized as occurring in the protozoa, but was supposed by Schaudinn to take place in the case of the malaria parasite and to be responsible for the relapses of that infection. These sporonts, still in the blood of the vertebrate host, develop into sexually differentiated cells (gametocytes), which are quiescent until they are removed from the blood; when this occurs both male and female cells extrude two polar bodies preparatory to the sexual process, and are then known as gametes. The flagella or microgametes extruded by the male cell function as spermatozoa, enter and fertilize the female cell (macrogamete), thereafter known as a zygote. After this act the zygote in some forms, especially the Plasmodidæ, becomes an elongated motile body (oökinete or travelling vermicule), which may remain free or become encysted (oöcyst). The oöcyst gives rise to secondary bodies in its interior (sporoblasts), which in turn develop into a number of minute rods known as sporozoites. Usually these sporozoites continue to enter the mouth parts of the insect, and by this route they once more gain the blood of the vertebrate host. The Protozoa are generally divided into four main classes.

Protozoa in which the protoplasmic body has no limiting envelope in the form of a stiff cortical layer, thus tending to a spherical shape in the floating forms, or an irregular, ever-changing shape in the creeping forms. Organs serving for locomotion and capture of food are known as pseudopodia; a skeleton or shell may be present. Encysted resistant forms occur which may give rise to a number of daughter cells.

Sexual conjugation (gametogamy} is the usual process of reproduction, though autogamy within the cyst wall has been described. EXAMPLES: Amœba limax, Entamœba coli. Entamœba histolytica.

Protozoa in which the organs of locomotion and for food capture are flagella, that is to say, long slender filaments capable of performing lashing movements.

EXAMPLES: Trichomonas, Lamblia, Trypanosoma.