CULTURES, AND THEIR STUDY. 203 



the peculiarities of certain pathogenic forms enable us 

 to use special means, taking advantage of their eccentrici- 

 ties, for their isolation, and that the general methods are 

 in reality more useful for the non-pathogenic than for the 

 pathogenic forms. 



All three methods depend upon the observation of 

 Koch, that when germs are equally distributed through- 

 out some liquefied nutrient medium which can be solidi- 

 fied in a thin layer, the growth of the germs takes place 

 in little scattered groups or families, called colonies, dis- 

 tinctly separated from each other and capable of trans- 

 plantation to tubes of culture-media. 



Plate-cultures. — The plate-cultures, originally made 

 by Koch, require considerable apparatus, and of late years 

 have given place to the more ready methods of Petri and 

 Von Esmarch. So great, however, is the historic interest 

 attached to the plates that it would be a great omission 

 not to describe Koch's method in full. 



Apparatus. — Half a dozen glass plates, about 6 by 4 

 inches in size, free from bubbles and scratches and 

 ground at the edges, are carefully cleaned, placed in a 

 sheet-iron box made to receive them, and then put in 

 the hot-air closet, where 

 they are sterilized. The box, 

 which is tightly closed, al- 

 lows the sterilized plates to 

 be kept on hand indefinitely 

 before using. 



A moist chamber, or double 

 dish, about 10 inches in di- 

 ameter and 3 inches deep, the 

 upper half being just enough 



larger than the lower to allow FlG - 2 3— Complete levelling appa- 

 , . . ,, ratus for pouring plate-cultures, as 



it to close over it, is carefully taught by Koch 



washed. A sheet of bibulous 



paper is placed in the bottom, so that some moisture can 



be retained, and a 1 : 1000 bichlorid solution is poured in 



and brought in contact with the side*, top, and bottom 



