158 BACTERIA. 



of the liquefied gelatine a greyish scum is found, in which 

 " involution forms " may be frequently observed. 



Cultivations made with a platinum needle on the surface 

 of agar-agar, grow as elevated pale translucent streaks, the 

 margins of which are well defined, and in the agar imme- 

 diately below the streak there occurs a slight opalescence 

 which is very characteristic of the bacillus. The agar, unlike 

 the gelatine, is not liquefied. As the growth becomes older 

 it spreads, though not very rapidly, the band increases in 

 thickness and gradually assumes a brown colour especially 

 in the opalescent substratum. When the organism grows in 

 meat infusion it forms a delicate greyish pellicle on the 

 surface, and in such a medium the movements of the bacillus 

 are most characteristic. If a single drop taken from the 

 surface of such a cultivation, or even a very small fraction of 

 a drop be added to a drop of meat broth faintly tinged with 

 a i in 2,000 solution of methyl violet and examined in the 

 moist chamber (see Appendix), the bacilli are seen to have 

 most rapid movements, especially if the temperature be 

 slightly raised ; Koch says, " when they accumulate in num- 

 bers at the border of the droplet, and are swimming about 

 actively, it seems quite as if they were a swarm of midges, 

 between which, diving here and there, are long spiral-shaped 

 threads, which are also moderately active." The shorter 

 rods become more and more curved, and then stretch them- 

 selves out, constantly moving, oscillating and twisting. 



From numerous experiments carried on by several observers 

 it appears that this motility is dependent partly on tempera- 

 ture, but chiefly on the desire for oxygen which these 

 organisms exhibit. In the " hanging drop " (see Appendix), 

 for instance, it will be seen that the cholera bacillus separates 

 itself from most other micro-organisms by its movements in 

 search of a supply of oxygen, which always bring it to the 

 margin of the drop ; once there, however, it loses its motility 

 and remains in its acquired position, where its wants 

 are supplied without any effort on its part. The cholera 

 bacillus also grows on potatoes, but here it should be noted 

 the growth does not go on at the ordinary temperature 

 of the room, but only when the temperature is raised 

 to 30 or 35 C. A fragment of blue litmus paper 

 placed on the surface of a slightly moistened boiled potato 

 indicates, by turning red, a slight acid reaction, which is due 



