Abstract
The characters by which the “R” variant of a given bacterial type is distinguished from the “S” variant are to-day widely known. Most of them are morphological and in most bacterial types it is not difficult to say whether a single colony is smooth or rough; but every bacteriologist is familiar with those borderline colonies, which are apparently smooth, but which later prove to be serologically rough. It sometimes happens that by special stimulation the reversion from an “R” culture into the “S” form succeeds, but that, notwithstanding the smooth appearance of the isolated colonies, the organisms persist in showing some characters which belong to the “R” variant, such as instability in saline (Bruce White, 1926).