552 APPLICATION OF METHODS OF BACTERIOLOGY 



ordinary way there is nothing characteristic about the 

 grouping; but in impression cover-slips made from young 

 cultures the short commas will nearly always be seen in 

 small groups of three or four, lying together in such a way 

 as to have their long axes nearly parallel to one another. 

 (See Fig. 88.) 



In old cultures in which development has ceased it under- 

 goes degenerative changes, and the characteristic comma 

 and spiral shapes may entirely disappear, their place being 

 taken by irregular involution-forms that present every 

 variety of outline. (See Fig. 89.) In this stage they take 

 on the stain very feebly, and often not at all. 



Cultural 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., develop- 

 ment can often be observed after as short a period as twelve 

 hours, but frequently not before sixteen to eighteen hours. 

 This is especially true of the first or "original" plate, con- 

 taining the largest number of colonies. At this time the 

 plate will present 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 which the colonies pass cannot, therefore, be studied 

 upon it. 



