GELATIN AND POTATO MEDIA. 183 



It has, consequently, given place to two modifications which 

 give excellent results. One or the other of these should, 

 therefore, be employed whenever it is desired to obtain 

 strictly pure cultures on potatoes. 



Esmarch potato cultures. — The cover of the Esmarch dish 

 (Fig. 28 d), is about 5 cm. in diameter, and the bottom is 2 

 cm. in height. These dishes, like those of Petri, are steril- 

 ized in the dry-heat oven (Fig. 25, p. 160), and then allowed 

 to cool. 



A small, sound potato is selected, and held with the 

 thumb and forefinger of the left hand. The outer edge of 

 the potato is pared circularly, by means of a clean potato- 

 knife which is held upright. Two horizontal sections, above 

 and below, are now made. The clean, sound core of the po- 

 tato, which is thus obtained, is slipped into a sterilized Es- 

 march dish. Each dish is thus supplied with a clean potato 

 section. The dishes are then placed in a steam sterilizer 

 and steamed for f to 1 hour. The potatoes will then be 

 sterilized and cooked. It is not necessary to heat the pota- 

 toes on three successive days. The potatoes will become 

 more or less dark, unless they are heated immediately after 

 they are cut. 



Laboratory work. — The student will prepare three of the Esmarch 

 potato dishes, and then make dilution cultures of the Bacillus prodi- 

 giosus. As little of the material as possible should be transferred, 

 each time, and it should be spread over the entire surface. The 

 method of inoculation is the same as that described on p. 168, except 

 that the potatoes are not taken out of the dishes. When dilution 

 cultures are not desired, a single mass culture may be made, or sev- 

 eral parallel streaks may be made by means of a platinum wire. In 

 the latter ca.se, the organism will develop along the streaks thus 

 made. Inasmuch as each potato is in a small sterilized dish by itself, 

 the risk of contamination, with careful and rapid manipulation, is 

 .very small. 



Test-tube cultures on potato. — In this method, the special 

 dish of Esmarch is done away with and its place is taken 



