448 BAGTEBIOLOGY. 



comma and spiral shapes may entirely disappear, their 

 place being taken by irregular involution-forms tliat 

 present every variety of outline. (See Fig. 76.) In 

 this stage they take on the stain very feebly, and often 

 not at all. 



CuLTUEAL Peculiarities. — On plates of nutrient 

 gelatin that have been prepared from a pure culture of 

 this organism and kept at a temperature of from 20° 

 to 22° C, development can often be observed after as 

 short a period as twelve hours, but frequently not be- 

 fore sixteen to eighteen hours. This is especially true 

 of the first or " original " plate, containing the largest 

 number of colonies. At this time the plate will pre- 

 sent to the naked eye an appearance that has been 

 likened to a ground-glass surface, or to a surface that 

 has been stippled with a finely pointed needle, or one 

 upon which very fine dust has been sprinkled. This 

 appearance is due to the presence of minute colonies 

 closely packed together upon the surface of the gelatin. 

 In the depth of the gelatin can also be seen closely 

 packed, small points, likewise representing growing 

 colonies. As growth progresses liquefaction occurs 

 around the superficial colonies, and in consequence this 

 plate is usually entirely liquid after from twenty-four 

 to thirty hours ; the developmental phases through Avliich 

 the colonies pass cannot, therefore, be studied upon it. 



On plates 2 and 3, where the colonies are more widely 

 separated, they can be seen after twenty-four to thirty 

 hours as small, round or oval, white or cream-white 

 points, and when located superficially a narrow trans- 

 parent zone of liquefaction can be detected around 

 them. As growth continues this liquefaction extends 

 downward rather than laterally, and the colony ulti- 



