PNEUMONIA. 349 The colonies which develop at 24° C. upon 15 per cent, gelatin plates are described as small, round, cir- cumscribed, finely granular white points which grow slowly, never attain any considerable size, and do not liquefy the gelatin (Fig. 99). If, instead of gelatin, agar-agar be used and the plates kept at the temperature of the body, the colonies which develop upon the plates appear as transparent, delicate, drop-like accumulations, scarcely visible to the naked eye, but under the microscope distinctly granular, the central darker portion being frequently surrounded by a paler marginal zone. In gelatin puncture-cultures, made with 15 instead of the usual 10 per cent, of gelatin, the growth takes place along the entire path of the wire in the form of little whitish granules distinctly separated from each other. The growth in gelatin is always very limited. Upon agar-agar and" blood-serum the growth consists of minute, transparent, semi-confluent, colorless, dew- drop-like colonies, which die before attaining a size which permits of their being seen without careful in» spection. In bouillon the organisms grow well, clouding the medium very slightly. Milk is quite well adapted as a culture-medium, its casein being coagulated. No growth can be secured upon potato at any tem- perature or by any manipulation yet known.1 When it is desired to maintain or increase the virulence of a culture it must be very frequently passed through the body of a rabbit. The degree to which the virtilence can be raised in this way is remarkable. C. W. Lincoln has succeeded in reducing the fatal dose for rabbits to f a c.cm. If a small quantity of a pure culture of the virulent 1 Ortmann asserts that the pneumococcus can be grown on potato at 37° 'C., but this is not generally confirmed. The usual acid reaction of the potato would indicate that it was a very unsuitable culture-medium.