74 BACILLUS DIPHTHERIA 



and adhere somewhat closely to the serum. On agar 

 and gelatine the characters of the colonies are as 

 described on serum, but they are more transparent. 

 Such gelatine cultures are seen in Figs. 69 and 70. 

 The former shows a copious growth of minute 

 colonies, and the latter isolated colonies in which 

 the characters indicated are well marked. 



Growths take place at temperatures between 20 C. 

 and 42 C., but are very slow at the lower limit ; 

 the growth on gelatine therefore takes some time 

 to develop. The optimum temperature is about 

 37 C. 



A preparation made from such a serum culture as 

 is described above is shown in Fig. 71, and in it will 

 be noted the clubbing and banding of the bacillus, 

 as well as the tendency to parallelism already 

 mentioned. 



A similar preparation from an agar culture is 

 represented in Fig. 72. As a rule, the bacillus from 

 agar, while it shows clubbed and dumb-bell forms, is 

 usually shorter and more regular in size than the 

 same organism grown upon blood serum, and does 

 not show such marked banding as these latter 

 cultures. In the photograph there are many so- 

 called involution forms, and the preparation shows 

 an unusual degree of irregularity. 



