METHODS OF PURE CULTURE 



47 



contain millions. Now, since only a loopful is taken from A for the 

 second inoculation, it follows that the number will be reduced to 

 hundreds only. When repeated from B to C, the latter will contain 

 only a very small number, perhaps about ten or less. The process so 

 far is thus practically a dilution method. Three sterilized Petri-dishes 

 (Fig. 61, A'B'C') are next placed in a row on the table. A Petri-dish 

 is a round glass dish from 6-10 cms. diameter, with glass sides about 

 1 cm. high. It has a glass cover which, as is seen in the figure, over- 

 laps the sides of the dish. The contents of the now inoculated gelatine 

 tubes are poured each into a separate Petri-dish, where the gelatine 

 cools and solidifies into a jelly. The three Petri-dishes are now put 

 away inside a glass dish covered by a bell-jar, and at the bottom of this 

 dish a thin solution of copper sulphate is usually placed to keep off the 

 moulds (Fig. 62). As the bacteria are encased in the Petri-dishes, and 

 these in their turn are encased in a glass house, 

 no bacteria from the outside can participate in the 

 food present in the nutrient gelatine. Now con- 

 sider what will happen. Each cell will divide, 

 the process of division being carried on by the 

 daughter-cells, so that in a day or two there will 

 be an immense number of individuals present. 

 But the case is different from that of a liquid 

 culture. Here the products of one cell remain 

 stuck together on account of the medium being 

 solid, therefore the gelatine, after about two 

 days, will be seen to be punctuated with a number of small dots, 

 each dot consisting of the descendants of one cell, and probably 

 containing about a million individuals. The Petri-dish which received 

 the contents of the gelatine-tube A will usually be found to have 

 so many of these dots that separation will be impossible. The same 

 is often the case with B. In C, however, the state of affairs is 

 different. Suppose that ten bacteria were contained in the gelatine, 

 they are poured out along with it into the Petri-dish. The gelatine 

 just covers the surface of the glass in the Petri-dish, and should 

 not be more than 1 mm. thick. Of course these ten are invisible, 

 but when each one multiplies to such an extent that where one 

 was present there are now a million, and when the million stick 

 together, the sum total becomes visible. If they all develop, ten 

 spots, usually round or oval, will be seen on the surface of the gelatine, 

 or perhaps some on the surface, others under it. These dots are called 

 Colonies. The next process is to touch a colony with a sterilised 



FIG. 62. 

 (For explanation see text.) 



