Although the hanging-drop method for the detection of bacterial motility can be employed successfully, the procedure has several distinct disadvantages. It is so tedious that the determination of this bacterial characteristic is frequently neglected in the routine laboratory. Furthermore, the results are often uncertain because it is difficult to observe motility when only a few of the cells in a culture are motile. Finally, it is necessary to provide relatively young cultures for the examination. The use of semi-solid media for the determination of bacterial motility, on the other hand, eliminates the shortcomings of a hanging-drop technique. The results are macroscopic. They are cumulative, thereby particularly qualifying the method for use in the routine laboratory, where examinations cannot always be carried out at a specific time. Moreover, this method practically eliminates the possibility of overlooking motility when only a small proportion of motile cells are present, because the localized out-growths, which occur wherever motile cells are deposited along the stab, can hardly escape notice. Semi-solid media have been employed for many years in the study of bacterial motility. Rosenthal (1895) reported marked differences in the size and shape of bacterial colonies which had developed in semi-solid nutrient gelatin. Klie (1896) called attention to the spreading and thread-like appearance of colonies 1 This work was supported in part by contributions from the General Education

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