The present study deals with the morphology and systematic position of the causative agent of infectious jaundice. There are several features which are not found in any of the hitherto known genera of Spirochætoidea which led me to give this organism an independent generic name, Leptospira, denoting the peculiar minute elementary spirals running throughout the body. The absence of a definite terminal flagellum or any flagella, and the remarkable flexibility of the terminal or caudal portion of the organism are other distinguishing features. Unlike all other so called spirochetes the present organism resists the destructive action of 10 per cent saponin.

A detailed comparative study of related genera, including Spirochæta, Saprospira, Cristispira, Spironema, and Treponema, has been given with the view of bringing out more strongly the contrast between them and the new genus.

A study has been made to discover whether any differential features exist among the strains of Leptospira icterohæmorrhagiæ derived from the American, Japanese, and European sources, but none has been found.

It is hoped that the creation of a new genus may facilitate a more exact morphological description than has hitherto been possible, due to the vague use of the term Spirochæta which indiscriminately covered at least six large genera of spiral organisms.

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