PLATE CULTIVATION 99 



tion, or the growth may become confluent owing to the 

 condensation water carrying the organisms all over the 

 film. 



The plate-culture method can be modified to suit par- 

 ticular circumstances. For example, the melted gelatin 

 or agar, uninoculated, may be poured into the dishes and 

 allowed to solidify. The film is then inoculated with 

 the material, by streaking or painting with an L-shaped 

 piece of glass rod or with a camel's hair brush, sterilised 

 by boiling, or by pouring a few drops of broth containing 

 the organisms upon it. This is the only way in which 

 blood-serum can be used, the sterile blood-serum being 

 placed in the Petri dish, solidified in the inspissator in 

 the same manner as for blood-serum tubes, and the 

 coagulated film inoculated. 



For many purposes plates are unnecessary, the same 

 result being obtained by rubbing over the surface of two or 

 three or more tubes of sloping agar or gelatin successively 

 the once charged needle, straight or looped. In the last 

 tube or two isolated colonies generally develop. 



The plate-culture method may fail if the organism to 

 be isolated forms but a small minority of the total 

 organisms present in the mixture ; the only alternative 

 then is to multiply the number of plates, which, however, 

 may entail great labour in their examination. 



Esmarch's roll cultures. Another modification of the 

 plate-culture method is known as Esmarch's roll culture. 

 For this purpose large test-tubes (" boiling tubes "), at 

 least an inch in diameter and 6 in. long, are sterilised 

 and plugged with cotton- wool. The sterile melted gelatin, 

 about 10 c.c., is poured in and inoculated, the wool plug 

 replaced, and the tube held in the horizontal position is 

 rotated under a stream of cold water, or in warm weather 

 on a block of ice, until the gelatin has set. In this way 

 the gelatin forms a thin film over the inside of the tube, 



72 



