464 PATHOGENIC AEROBIC BACILLI 



temperature more rapidly in the incubating oven. Grows in a de- 

 cidedly acid medium. 



In gelatin plates colonies are developed in from twenty-four to 

 forty-eight hours, which va,ry considerably in their appearance ac- 

 cording to their age, and in different cultures in the same medium. 

 The deep colonies are usually spherical and at first are transparent, 

 homogeneous, and of a pale-straw or amber color by transmitted 

 light ; later they frequently have a dark-brown, opaque central por- 

 tion surrounded by a more transparent peripheral zone ; or they may 

 be coarsely granular and opaque ; sometimes they have a long-oval 

 or "whetstone" form. The superficial colonies differ still more in 

 appearance ; very young colonies by transmitted light often resemble 

 little drops of water or fragments of broken glass ; when they have 

 sufficient space for their development they quickly increase in size, 

 and may attain a diameter of three to four centimetres ; the central 

 portion is thickest, and is often marked by a spherical nucleus of a 

 dark-brown color when the colony has started below the surface of 

 the gelatin ; the margins are thin and transparent, the thickness 

 gradually increasing towards the centre, as does also the color, which 

 by transmitted light varies from light straw color or amber to a dark 

 brown. The outlines of superficial colonies are more or less irregular, 

 and the surface may be marked by ridges, fissures, or concentric 

 rings, or may be granular. The writer has observed colonies re- 

 sembling a rosette, or a daisy with expanded petals. Escherich 

 speaks of colonies which present star-shaped figures surrounded by 

 concentric rings. 



In gelatin stick cultures the growth upon the surface is rather 

 dry, and may be quite thin, extending over the entire surface of the 

 gelatin, or it may be thicker with irregular, leaf -like outlines and 

 with superficial incrustations or concentric annular markings. An 

 abundant development occurs all along the line of puncture, which 

 in the deeper portion of the gelatin is made up of more or less closely 

 crowded colonies ; these are white by reflected light, and of an am- 

 ber or light-brown color by transmitted light ; later they may become 

 granular and opaque. Frequently a diffused cloudy appearance is 

 observed near the surface of the gelatin, and under certain circum- 

 stances branching, moss-like tufts develop at intervals along the line 

 of growth. One or more gas bubbles may often be seen in recent 

 stick cultures in gelatin. 



Upon nutrient agar and blood serum, in the incubating oven, an 

 abundant, soft, white layer is quickly developed. Upon potato an 

 abundant, soft, shining layer of a brownish-yellow color is developed. 

 The growth upon potato differs considerably, according to the age of 

 the potato. According to Escherich, upon old potatoes there may 



