KOCH'S PLATE CULTURES. 131 



externally, are steeped for an hour in a i per mil solution of subli- 

 mate, then swilled with water and sterilised in a wire basket by two 

 hours' exposure in a current of steam. When this is effected, and 

 they are so far re- cooled as to be fit for handling (with disinfected 

 fingers), they are cut into halves by a sterilised knife and placed 

 under a sterilised bell-^glass. When cold, inoculating streaks are 

 drawn on the cut surfaces, and subsequently develop into potato 

 cultures. 



85. Koch's Plate Cultures 



are, as previously indicated, prepared by pouring out the liquefied 

 and inoculated medium (e.g., 5-8 c.c. in a test-tube) on to colourless 

 glass plates, rectangular in form, about one-twelfth of an inch thick, 

 5 to 6 inches long, and 3^ to 4 inches broad, previously sterilised 

 in batches in a copper or iron box from which they are taken as 

 required. The plates are laid on a plate-pouring apparatus arranged 

 horizontally as described and shown in the above-named hand- 

 books and the distribution of the stratum of gelatin or agar-agar 

 is assisted by the aid of the rim of the test-tube. To sterilise the 

 latter, it should be held for a short time in the Bunsen flame and 

 allowed to re-cool sufficiently before proceeding to pour. When 

 the gelatin layer is set the plate is transferred to a sterile damp 

 chamber, which is placed in the thermostat and maintained at the 

 constant temperature required. 



These plates are rather inconvenient to handle, since, in follow- 

 ing up the development of the growing colonies, the plate must 

 be frequently taken out of the chamber. During each observation 

 the mould spores in the air are liable to fall upon the medium, 

 where they rapidly develop into such masses of branched threads 

 that the bacterial colonies are smothered, thus rendering all the 

 care bestowed upon the preparation of no avail. In order to pre- 

 vent this, the flat plates are replaced by double shallow glass 

 dishes, in which the cultures can be examined under low powers 

 without being exposed to the air. These dishes were first intro- 

 duced into bacteriology by Salomonsen, but are generally known 

 in Germany as Petri dishes, this latter worker having been the 

 first to test them. Their use for this purpose can be recommended. 

 Instead of pouring out the inoculated gelatin, the closed tube can 

 be held almost horizontally under the stream from the water-tap 

 and slowly turned round on its axis, whereby the contents are 

 distributed uniformly over the walls, and will set as a thin stratum 

 wherein the germs then develop into colonies. These cultures 

 are generally called Esmarch tubes or roll cultures, and were 

 first proposed by W. Hesse. 



Plate cultivation affords useful assistance, not only for the 

 separation of a bacterial mixture into its several species, but also 

 for the determination of the number of cells present therein, a 



