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XX] In bouillon the cultures present a characteristic appearance the liquid remains clear, whilst a granular deposit takes place on the sides and bottom of the tube. Cultivated on brothin which clarified butter or coco-nut oil is floated, B.pestis presents characteristic stalactite growths which gradually fall off, forming a granular deposit. Examined with the microscope, these various cultures show chains of a short bacillus, presenting here and there large bulbous swellings. In gelatin the bacilli sometimes form fine threads, sometimes thick bundles made up of many laterally agglomerated bacteria. The bacillus does not produce spores. The most favourable temperature for culture is from 36° to 39° C.

Intensification and attenuation of virus. —There can be no question that, both by artificial means and in a natural way, the virulence of the bacillus of plague is susceptible of modification. It has been remarked in Russia, in Persia, and in Calcutta that certain outbreaks of plague were preceded by a sporadic or epidemic febrile, sometimes afebrile, affection, in the course of which the lymphatic glands became enlarged and perhaps suppurated. Cases of this nature, in which a bacteriological examination proved that a cocco-bacillus was present in the blood and enlarged glands, are recorded by Drs. Cobb and Simpson. It is further known that in some instances the virulence and case-mortality of a plague epidemic show a tendency to decrease, the early cases being the most frequently and most rapidly fatal.

Recent observations in India have brought to light a form of chronic plague in rats, in which the specific bacterium, without seriously affecting the health of the animal, remains latent for long periods in abscesses in connection with spleen, liver, or abdominal lymphatics. Although thus latent, it is potentially virulent, proving lethal when cultures obtained from these abscesses are injected into other animals. These facts seem to indicate that under certain unknown natural conditions the virus tends to acquire increased potency, whilst in other circumstances its virulence tends to diminish.

This conjecture is countenanced by the results of experiment on animals. It has been shown that by