PETRI S DISHES. 69 



and distributed by means of a sterile wire over the surface, 

 care being taken not to bring the gelatine too near the 

 edge. The glass cover is then lowered, and other plates 

 can then be prepared in the same way by placing them on 

 a metal or glass rack over the first plate. 



Petri's Dishes. — The use of these has some advantages 

 over the plate method of Koch. The dishes are from 10 to 

 20 centimetres wide and about 1'5 or 2 centimetres deep, 



Fig. 14. — Petri Double Dish. 



and have roughly-fitting covers of the same form as the 

 dishes themselves. These dishes can be safely carried 

 about, and do not need the levelling apparatus ; moreover, 

 the colonies may be examined and counted if desired with- 

 out removing the lid, and, consequently, without the 

 exposure to contamination to which Koch's form of plate 

 is liable. Petri's double dishes are made both in the round 

 and square form. 



Esmarch's Roll Cultures. — A useful modification of Koch's 

 method is that of Von Esmarch. Instead of pouring the 

 liquid gelatine medium upon plates or in shallow dishes, it 

 is distributed in a thin layer upon the walls of a wide test- 

 tube. This is done by rotating the tube upon a block of 

 ice or in iced water. It is more convenient to turn the 

 tubes upon a block of ice having a horizontal surface, in 

 which a shallow grove is first made by means of a test-tube 

 containing hot water. In the winter the tube can be re- 

 volved under the water service tap to solidify the jelly. 



A little practice will enable the operator to distribute the 

 jelly in an even layer on the walls of the tube, and as 



