APPEARANCES OF CULTURES 361 



the diversity in the length of the bacilli is such as to throw 

 doubt on the purity of the culture. As a general rule, in a 

 young (twenty-four or forty-eight hours old) culture, grown at a 

 uniform temperature, the bacilli are plump, and the protoplasm 

 stains uniformly. In old cultures, or in cultures which have 

 been exposed to changes of temperature, the protoplasm stain's 

 only in parts ; there may be an appearance of irregular vacuola- 

 tion either at the centre or at the ends of the bacilli. 



Fig. 107. — Typhoid bacilli, from a young culture on agar, showing 

 flagella. See also Plate III., Fig. 15. 

 Stained by Van Ermengem's method, x 1000. 



Motility. — In hanging-drop preparations the bacilli are found 

 to be actively motile. The smaller forms have a darting or 

 rolling motion, passing quickly across the field, whilst some show 

 rapid rotatory motion. The filamentous forms have an undu- 

 lating or serpentine motion, and move more slowly. Hanging- 

 drop preparations ought to be made from agar or broth cultures 

 not more than twenty-four hours old. In older cultures the 

 movements are less active. 



Flagella. — On being stained by the appropriate methods 



