TECHNIQUE OF MAKING PLATES. 145 



redissolved at a temperature not detrimental to the 

 organisms with which the tubes may have been inocu- 

 lated. 



THE COOLING-STAGE AND LEVELLING-TRIPOD. 

 While the medium of which the plates are to be made 

 is melting it is well to arrange the cooling-stage (Fig. 

 25) upon which the gelatin or agar-agar is to be subse- 

 quently solidified. 



This stage consists of a glass dish filled with ice- 

 water and covered with a ground-glass plate, which in 

 turn has a dome-shaped cover. The dish rests upon a 

 tripod which can be brought to an exact level, as indi- 

 cated by the spirit-level, by raising or lowering its legs 

 by means of thumb-screws, with which they are pro- 

 vided. Three stages are usually employed. When 

 ready for use they should be exactly level. 



THE GLASS PLATES. In the original plate method 

 devised by Koch the contents of each of the tubes of 

 agar-agar or gelatin are poured out in a thin layer upon 

 the surface of a sterile glass plate. As soon as pre- 

 pared, the glass plates containing the culture are placed 

 in a large glass culture dish. 



PETRI'S MODIFICATION OF THE PLATE METHOD. 

 The modification, now in general use, that approaches 

 nearest to the original method, and at the same time 

 lessens very materially the number of steps in the proc- 

 ess, is that suggested by Petri. It consists in substitut- 

 ing for the plates small, round, double glass dishes, 

 having about the same surface-area as the plates (Fig. 

 26). The liquid medium is poured directly into these 

 little dishes and their covers replaced ; they are then 

 set aside for observation. In all other respects the 

 process is the same as Koch's original method. Petri's 

 10 



