THE METHODS OF INOCULATING SOLID MEDIA. 71 



evenness of surface in the gelatin on the glass plates a levelling- 

 tripod is used, as seen in Fig. 32. This tripod is easily set 

 by means of a levelling glass. 



Results. In this manner each of the separate bacteria con- 

 tained in the media will develop as separate colonies on the 

 gelatin plates. Plate I M made from tube L, will contain a 

 large number of bacteria ; plate II. will contain the bacteria 

 in much smaller number; and plate III. will contain only a 

 few colonies, well separated ; and in this way the characteristic 

 growth of the separate colonies and their action on gelatin 

 may be carefully followed out and studied. By this means 

 also the observer may under a magnifying-glass with a fine 

 sterile platinum needle pick out the individual colonies and 

 inoculate fresh tubes, and so obtain pure cultures of any liv- 

 ing organism. 



3. Plate cultures of agar are made in the same way, but 

 require more care in their preparation, as agar does not melt 

 at a temperature below 42 C., and solidifies again at a tem- 

 perature of 39 or 40 C., so that after inoculation in the 

 liquid state the tubes must be kept in a water-bath between 

 40 and 42 C. until they are poured into the plates. The 

 agar plates, however, may be incubated at blood tempera- 

 ture (37 C.), and the growth of bacteria noted. 



In recent years small double dishes, known as Petri's culture- 

 dishes (Fig. 22), have been introduced to take the place of 

 the plates. For the inoculation, liquefied gelatin or agar is 

 poured into the lower dish, and this is quickly covered by 

 the upper dish. For this method no levelling apparatus and 

 cooling-stage are required. 



For counting the colonies in the plates the apparatus of 

 Wolfhuegel (Fig. 31) has been adopted. 



This consists of regularly lined glass plates, divided into 

 squares and arranged as seen in the figure. By placing the 

 culture-plate under the lined plate it is easy to ascertain the 

 number of colonies contained in each of, for example, ten 

 squares, and by a simple process of multiplication the number 

 of colonies in the whole plate. 



4. Instead of pouring out the liquid gelatin from the tubes 



