BUBONIC PLAGUE. 437 istic of the plague bacillus. The peculiarities of these colonies cannot be recognized after forty-eight hours. Involution-forms on partly desiccated agar-agar not con- taining glycerin are said by Haffkine to be characteristic. The microbes swell up and form large, round, oval, pea- or spindle-shaped or biscuit-like bodies, which may attain twenty times the normal size and in growing gradually lose the ability to take up the stain. Such involution- forms are not seen in liquid culture. Hankin and Leumannl recommend for the differential diagnosis of the plague bacillus the addition of 2.5-3.5 per cent, of salt to the agar-agar. When transplanted from ordinary agar-agar to the salt agar-agar the involu- tion-forms which are so characteristic of the plague ba- cillus form with exceptional rapidity. Upon blood-serum the growth at the temperature of the incubator is luxuriant It forms a moist layer of a yellowish-gray color, and is unaccompanied by liquefac- tion of the serum. Upon potato no growth occurs at ordinary tempera- tures. When the potato is stood away for a few days in the incubator a scanty, dry, whitish layer develops. Abel found the best culture-medium to be 2 per cent, alkaline pepton solution with i or 2 per cent, of gelatin, as recommended by Yersin and Wilson. The bacillus develops under conditions of aerobiosis and anaerobiosis. In glucose-containing media it does not form gas. No indol is formed. Ordinarily the culture- medium is acidified by the development of an acid that persists for three weeks or more. By frequent passage through animals of the same species the bacillus increases very much in virulence. Curiously enough, however, the observations of Knorr, substantiated by-Yersin, Calmette and Borrel, show that the bacillus made virulent by frequent passage through mice is not increased in virulence for rabbits.2 1 Centralbl. /. Bakt. u. Parasitenk., Oct., 1897, Bd. xxii., Nos. 16 and 17, p. 438. 2 Ann. de VInst. Pasteur, July, 1895.