98 TEXT-BOOK OF BACTERIOLOGY. 



difficult. Efforts have been made to overcome these difficulties, 

 two different ways having- been suggested. 



In one of the modifications of Koch's method, flat-covered dishes, 

 as recommended in different forms by Babes, Sayka, Petri, and 

 others, take the place of the ordinary glass plates. The pattern 

 introduced by the last-named investigator is the one generally em- 

 ployed, in which the lower dish is just 10 cm. in diameter. 



A number of such dishes are sterilized in the hot-air chamber 

 and then prepared for use. When the dilutions have been made in 

 the usual manner, the cover of a dish is removed, the liquid gelatin 

 poured from the test-tube into the dish and distributed over the 

 bottom of it by a suitably tipping or slanting of the dish, which is 

 covered again, and the gelatin left to harden. The dishes thus 

 prepared are piled one above another, and in due time the develop- 

 ment of germs and the growth of colonies occur. 



The advantages of this system are great. The manipulation is 

 much simpler and more convenient. The difficulties which a be- 

 ginner usually has to contend with are, for the most part, avoided ; 

 the gelatin spreads, almost of itself, in an even layer over the sur- 

 face, and hardens there quickly and perfectly, without the aid of 

 the levelling apparatus; the often troublesome after-pollution of 

 the glass plates by adventitious germs falling- upon them is here 

 prevented almost to a certaint}^ by the protecting- cover; the 

 contact of the fingers with the layer of gelatin, which is apt to 

 occur in removing and examining the plates, cannot take place, and 

 the extensive liquefaction of the gelatin, the " running over " of the 

 plates, is also impossible. 



Most of the apparatus employed in Koch's original system be- 

 comes unnecessary and superfluous. Glass plates, plate-box, level- 

 ling apparatus, glass bridges, and the large bell-glass can be 

 dispensed with, and when it is further considered how easily the 

 "dish-plates" may be removed and transported, we cannot but 

 acknowledge the superiority of this process in many respects. 



For some purposes, however, a second modification of Koch's 

 glass-plate process, introduced by E. von Esmarch, has still greater 

 advantages. 



The liquid g-elatin is inoculated in the usual wa}^ but then, in- 

 stead of being poured out of the test-tube, it is spread over the 

 inner walls of the test-tube itself and there left to harden. The 

 interior surface of the tube becomes covered with a thin, even layer 

 of gelatin, which possesses about the same extent of surface as that 

 obtained in the case of the ordinary glass plates. The germs de- 

 velop precisely in the same time and manner as on the plates; it 



