NOT DESCRIBED IN PREVIOUS SECTIONS. 563 
Yersin, who was sent by the French Government to study the 
bubonic plague at Hong-Kong, arrived in that city on the 15th of 
June, 1894. He describes the bacillus found in the contents of the 
buboes as being short and thick, with rounded ends, staining easily 
with the aniline colors, butnot by Gram’s method. “The extremities 
stain more intensely than the centre, so that they often present a 
clear space in the middle. Sometimes the bacilli appear to be sur- 
rounded by a capsule. . . . In bouillon the bacillus has a very char- 
acteristic appearance, resembling the cultures of the streptococcus of 
erysipelas—a clear liquid with grumous deposits on the walls and at 
the bottom of the tube. These cultures examined under the micro- 
scope show veritable chains of short bacilli, presenting in places a 
considerable spherical enlargement.” 
This bacillus is sometimes seen to be motile, and it has flagella, 
which, however, are difficult to stain (Gordon). 
In agar cultures, in the incubator at 37° C., involution forms soon 
appear. These may be spherical, oval, pyriform, etc., and are often 
many times larger than the typical bacillus. 
Biological Characters.—We quote from Kitasato’s preliminary 
report as follows: 
The bacilli show very little movement, and those grown in the incubator, 
in beef-tea, make the medium somewhat cloudy. The growth of the bacilli is 
strongest on blood serum at the normal temperature of the human body 
(34° C.); under these conditions they develop luxuriantly and form a col- 
ony moist in consistence and of a yellowish-gray color; they do not liquefy 
the serum. On agar-agar jelly (the best is good glycerin agar) they also 
grow freely. The different colonies are of a whitish-gray color and by re- 
flected light have a bluish appearance ; under the microscope they appear 
moist and in rounded patches with uneven edges ; at first they appear every- 
where as if piled up with ‘‘glass-wool,” later as if having dense, large cen- 
tres. If acover-glass preparation is made from a cultivation on agar-agar, 
and, after having been stained, is observed under the microscope, long 
threads of bacilli are seen, which might, by careless inspection, be mistaken 
for a coccus chain, but are recognized with certainty as ‘‘ threads of bacilli” 
under closer observation. The growth on agar-gelatin is similar to that on 
agar-agar ; in a puncture cultivation at the ordinary temperature after a few 
days they are found growing as a fine dust in little points alongside the 
puncture, but with very little growth on the surface. Whether these ba- 
cilli are able to liquefy ordinary gelatin or not Iam at present unable to de- 
cide, as the temperature of Hong-Kong ranges so high that the employment 
of simple nutritive gelatin is out of the question. I shall give further infor- 
mation on this question later. On potatoes at a temperature of from 28° 
to 30° C., there was no growth after ten days’ observation, but at a tempera- 
ture of 37° C. the bacilli developed sparingly after a few days; the growth 
was whitish-gray in color and exsiccated. As mentioned before, the bacilli 
grow best at a temperature of from 38° to 39° C.; at how low a temperature 
growth is possible Iam unable at present to state. So far 1 have been un- 
able to observe the formation of spores. 
Experiments on Animals.—Mice, rats, guinea-pigs, and rabbits are sus- 
ceptible to inoculation. If these animals are inoculated with pure culti- 
vations, or with the blood of a plague patient in which the bacilli have been 
