THE TYPHOID BACILLUS. 207 



characteristic growth of the typhoid bacillus upon 

 gelatine. These plates are then put into the incuba- 

 tor and kept at a temperature of from 20° to 22° C. On 

 the second day the development of the colonies has 

 proceeded so far that they can be seen with the naked 

 eye ; twenty-four hours later it can be further seen 

 that they are not all alike. Some have spread them- 

 selves out on the surface, forming a translucent, al- 

 most transparent mass, which lies on the gelatine like 

 a delicate coating with uneven edges. From the 

 centre of such a colony there are radiating lines some 

 of which reach to the circumference, whilst others 

 stop half way ; sometimes again they start midway 

 between the centre and the periphery, and stretch up 

 to the latter. In a similar manner there are lines 

 which run parallel to the circumference, sometimes 

 right round the colony, and sometimes only for part 

 of the way ; these always keep the direction, but like 

 the radiating lines are without a fixed arrangement. 

 Thus the colonies have a most peculiar appearance. 

 Gradually they grow larger and larger, and by de- 

 grees lose at their centres this characteristic appear- 

 ance, which however is still to be seen at the edges. 

 Many minerals exhibit a similar structure at the point 

 of fracture, which is then described as a conchoidal 

 fracture. These superficial colonies, under favourable 

 circumstances, especially when they are only a few on 

 each plate, become more than I cm. in diameter. 



