36 THE AMERICAN MONTHLY [Feb 
Upon glycerin-agar the development of the colonies is 
slower, though in the end the colonies attain'a larger size 
than those grown upon plain agar. Klein says that the 
colonies develop quite readily upon gelatin made from 
beef-bouillon (not infusion), appearing in twenty-four 
hours at 20° C. as small gray, irregularly rounded dots. 
Magnification shows the colonies to be serrated at the 
edges and made up of short, oval, sometimes double ba- 
cilli. Some colonies contrast markedly with their neigh- 
bors in that they are large, or looped threads of bacilli. 
The appearance was much like that of the young colon- . 
ies of the Proteus vulgaris. At first these were regard- 
ed as contaminations, but later he was led to believe that 
their occurrence was characteristic of the plague bacillus. 
The peculiarities of these colonies cannot be recognized 
after forty-eight hours. Involution-forms on partly de- 
siccated agar-agar not containing 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 Leumann recommend for the differ- 
ential diagnosis of the plague bacillus, the addition of 2.5 
to 3.5 per cent of salt to the agar-agar. When transplant- 
ed from the ordinary agar-agar to the salt agar-agar, the 
involution-forms which are socharacteristic of the plague 
bacillus, form with exceptional rapidity. In bouillon with 
this high percentage of salt, the stalactite formation is 
very beautiful and characteristic. Upon blood-serum, the 
growth at the temperature of the incubator is luxurient. 
It forms a moist layer of a yellowish-gray color, and is 
unaccompanied by liquefaction of the serum. Upon po- 
tato, no growth occurs at ordinary temperature. 
When the potato is put for a few days in the incubator, 
a scanty, dry, whitish layer develops. Abel found the 
