From the foregoing experiments, the conclusion may be drawn that B. bifidus communis of Tissier has an aerobic phase, in which it resembles B. mesentericus fuscus. Numerous intermediate phases can occur between these two extremes; and their morphological and biological variabilities demand the utmost attention in order to interpret more intelligently the various phases of a given organism, constantly found in the stools of sucklings, and to avoid the artificial creation of two or more organisms from a single microbic type.