NUTRIENT MEDIA AND METHODS OF CULTIVATION. 105 



After the final treatment in the steam steriliser some of the 

 of gelatine and agar-agar are placed upright and allowed to 

 set, and others are placed on an inclined plane or in the blood-serum 

 in.Npissator, and left to gelatinise with an oblique surface. 



(B) METHODS OF EMPLOYING NUTRIENT JELLY IN TEST-TUBES 

 AND ON GLASS PLATES. 



Test-tube-cultivations. To inoculate test-tubes containing 

 nutrient jelly, the cotton- wool plug is first twisted round in case 

 there are any adhesions between the plug and the test-tube. It is 

 then removed with the thumb and 

 index finger of the right hand, and 

 placed between the fourth and fifth 

 fingers of the left hand, instead of 

 being put down on the laboratory table 

 and thereby probably contaminated 

 with bacteria or the spores of mould 

 fungi. A sterilised needle charged, 

 for example, with blood or pus con- 

 taining bacteria, or with a colony from 

 a plate-culture, is thrust once in the 

 middle line into the nutrient jelly, 

 and steadily withdrawn. The tube 

 should be held horizontally or with 

 its mouth downward, to avoid, as far 

 as possible, accidental contamination 



from the gravitation of germs in the 



j , , . i i FIG. 35. METHOD OF IXOCULAT- 



air; and the plug replaced as quickly ^ A TJMJT . TCBB COXTAINING 



as possible. The cotton- wool project- STERILE NUTRIENT JELLY. 

 ing beyond the mouth of the tube is 



then thoroughly burnt in the flame of a Bunsen burner or blow- 

 pipe, and an india-rubber cap fitted over the mouth of the tube 



The chances of error arising from contamination of the culti- 

 vations are reduced by avoiding draughts at the time of inoculation, 

 and it is best that these manipulations should be carried on in a 

 quiet room in which the tables and floor are wiped with damp cloths, 

 rather than in a laboratory in which the air becomes charged with 

 germs through constant sweeping and dusting, and the entrance 

 and exit of classes of students. In conducting any investigation 

 a dozen or more tubes should be inoculated, and if by chance an 

 adventitious germ, in spite of all precautions, gains an entrance, 



