vasion of the body from wounds by the bacillus pyocyaneus have not been observed" (Welch).

Bacillus of Bubonic Plague.-Morphology. In the tissues the organism occurs as a medium-sized short bacillus with rounded ends. In cultures its size and length vary and its median portion may be swollen so that an ovoid form is produced; it may grow in pairs and in chains, and it may occur as long, thread-like forms. Involution forms of elliptical or round shape, and often of large size, sometimes resembling yeast-cells, are frequent in old cultures or in cultures on special media. These involution forms are easily produced by cultivation on agar-agar containing 2% to 3%1⁄2 per cent. of sodium chlorid.

Staining. The organism stains with the usual aniline dyes, and is decolorized by Gram's method of staining. In the tissues it stains more deeply at its extremities than at its central portions, and it sometimes appears to possess a capsule. The polar staining may sometimes be brought out in cultures by weak staining solutions or by decolorization by alcohol. It is not motile, and it does not form spores.

Gelatin Plates.-The colonies on the surface appear after twenty-four to forty-eight hours at 22° C. They are flat, round, and white or yellowish white in color. Under a low magnifying power the central portion of the colony is gran

ular, while the marginal portion is clear. The colonies do not spread over the surface of the medium.

Gelatin Stab.-Growth all along the line of inoculation with the formation of a layer of growth at the surface of a whitish color. There is no liquefaction of the gelatin.

Gelatin Slants.-A whitish or slightly yellowish layer presenting nothing characteristic.

Agar-agar Plates.-The colonies on the surface appear first as dew-drops, and have already attained their maximum development after twenty-four to forty-eight hours in the

FIG. 73.-Bacilli of plague and phagocytes, from human lymphatic gland; x 800 (Aoyama).

incubator. They will then grow white in color, and present an opalescent or iridescent margin. Under the microscope they are distinctly granular. Considerable difference in size may be observed among the colonies. The larger colonies are said to be less virulent for animals than the smaller colonies, and it is claimed that these larger colonies when transplanted give rise to large colonies again.

Agar-agar Slant.-The colonies tend to become conflu ent, and the growth is somewhat viscid.

Bouillon.-The fluid usually remains clear, and the growth appears in the form of a granular or flocculent sediment

which may here and there adhere to the wall of the tube. In bouillon cultures richly inoculated and retained in a perfectly and undisturbed position at room-temperature for some days a characteristic appearance is produced. In twenty-four to forty-eight hours islands of growth appear underneath the surface in the form of flakes. In the next twenty-four to forty-eight hours there grow down into the fluid from the flakes long, stalactite-like masses, the liquid remaining clear. In four to six days the islands of growth have become more compact and solidified. If the flask be now slightly disturbed, the islands fall to the bottom, bringing with them the stalactite-like growths. The latter are very fragile. In addition to these appearances there is a deposit of growth on the wall of the flask and at the bottom, as well as a ring of growth on the margin of the surface of the liquid.

Milk.-Growth without coagulation.

No production of indol.

In neutral litmus bouillon the blue color is changed to red. There is no odor, and no pigment production.

The organism is aërobic.

It remains alive in cultures for five to six weeks at least. Growth occurs at all temperatures from 4° C. to 37° C. The best temperature for growth is 30° to 32° C.

Pathogenesis. The organism is pathogenic for a great variety of animals, including mice, rats, guinea-pigs, and rabbits. In these animals death generally follows in from two to six days after subcutaneous inoculation. The lesions produced are hemorrhagic edema at the seat of inoculation, enlargement of the lymphatic glands with more or less hemorrhage, enlargement of the spleen and its follicles. The bacilli are present in large numbers in the enlarged lymphatic glands and in the internal organs; they are less numerous in the blood. Rats and certain other animals may be infected by feeding. Pigeons, chickens, and cattle are immune.

Occurrence. The bacillus is found in large numbers in the buboes, pustules, pulmonary lesions, and other localized

lesions of the bubonic plague. It also may be found in larger or smaller numbers in the blood and internal organs generally, and it may be present in the sputum, bile, and alvine discharges. The pus of the buboes which break spontaneously may be sterile. The organism may be demonstrated in the circulating blood of cases of plague.

Bacteriological Diagnosis.-In cases of suspected plague the bacillus is to be sought for in the blood and in the buboes. In cases of pneumonia the sputum especially is to be examined. In the examination cultures as well as cover-glass preparations are to be used.

Perhaps the most certain method of identification of the bacillus is the inoculation of the mucous membrane of the nose of the rat. The simple rubbing of a portion of the culture upon the mucous membrane appears to be sufficient to produce a fatal result in the rat if the culture is that of the genuine bacillus. As a culture-medium agar-agar or blood-serum is to be used in cases where there is no mixed infection. If there is mixed infection of the material to be examined, gelatin surface-cultures are to be made.

The inoculation of animals for diagnostic purposes should be made with the greatest precaution to prevent the spread of the disease.

Bacillus of Influenza.'-Morphology.-Very small bacilli, with rounded ends and of variable length, sometimes growing into long forms, more or less bent or curved (see Fig. 74).

Stains more deeply at the ends than in the middle, and in the long forms shows irregularity of staining. The faintly stained areas are very sharply defined, as in the case of the typhoid bacillus.

In cover-glass preparations from bronchial secretions (see Fig. 75). the bacillus appears smaller and less plump than it does in preparations from cultures. It also does not show irregularities in staining.

Cultivation.-Does not grow in the ordinary culture-media, but may be cultivated on agar-agar "slants," the surfaces 1 Pfeiffer Zeitschrift f. Hygiene u. Infectionskrankheiten, Bd. 13, 1893.

:

of which have been smeared with a few drops of sterile

FIG. 74. Influenza bacilli from a culture on blood-agar; X 2000 (Wright and

FIG. 75.-Bacilli of influenza in a leucocyte in a cover-glass preparation from sputum. A pneumococcus also in the same leucocyte and other pneumococci free. The small size of the bacillus of influenza will be apparent by comparison with the pneumococci; X 2000 (Wright and Brown).

blood. The blood of man, rabbits, guinea-pigs, pigeons, or frogs will serve for this purpose, the best growth being