The invariable presence of a rickettsia in the gonads of the mosquito,Culex pipiens, is confirmed. This rickettsia is found in the cytoplasm of every germ cell in both sexes at all stages of development from larva to adult. No other species of mosquito is known to be similarly infected.The morphology of the organism shows the following features. In addition to “typical rickettsiae”, i.e. minute rods and coccoids which multiply by binary fission, there is a variety of other forms which are interpreted as a series of stages, usually in the following sequence: small coccoids become enlarged, and instead of dividing by binary or multiple fission, undergo a condensation of their stainable substance into points or rods which ultimately appear as various combinations of sharply curved rods and coccoids retained as in a matrix within the body of the original or “parent” coccoid. As the matrix disappears, they become the irregularly curved forms and compact aggregations of “daughter” forms characteristic of this rickettsia.The pathological germ cells previously noted in the testis by cytologists were found to occur in both sexes at all stages, and to be apparently the result of infection with this rickettsia.The problem of classifying the rickettsiae is discussed. The similarity of the rings and curved forms of various rickettsiae to corresponding forms, as yet little studied, in other groups of organisms is pointed out.The nameWolbachia pipientis(gen. et sp.n.) is proposed for the rickettsia ofCulex pipiens.Large, homogeneous or granular cell inclusions with an affinity for neutral red, termedNR bodies, were found in the gonad wall of nearly every specimen ofCulex pipiens, but were not found inC. territans. They resemble the inclusion bodies of certain viruses and at times closely simulate masses of rickettsiae, but are apparently distinct from the species here described. Their nature is unknown but their appearance and behaviour are consistent with their being living agents or the manifestations thereof. A degenerative process, in which NR bodies replace primary ovarian follicles in whole or in part, is described.Inoculation and cultivation experiments resulted negatively for both rickettsiae and NR bodies.

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