72 METHODS OF CULTIVATION OF BACTERIA. 



which develop when that amount is added to a tube of suitable 

 medium, and the latter plated and incubated. An ordinary 



plate should be used in such 

 a case, and the medium poured 

 out in as rectangular a shape 

 as possible. For the counting, 

 an apparatus such as is shown 

 in Fig. 37 is employed. This 

 consists of a sheet of glass ruled 

 into squares as indicated, and 

 supported by its corners on 



FIG. 37. Apparatus for counting colonies 



(Wolff hugei's). wooden blocks. The table to 



which these blocks are attached 



has a dark surface. The plate-culture containing the colonies 

 is laid on the top of the ruled glass. The number of colonies 

 in, say, twenty of the smaller squares, is then counted, and 

 an average struck. The total number of squares covered by 

 the medium is then taken, and by a simple calculation the 

 total number of colonies present can be obtained. Plate-cul- 

 tures in Petri's dishes are sometimes employed for purposes of 

 counting. The bottoms of such dishes are, however, never 

 flat, and the thickness of the medium thus varies in different 

 parts. If these dishes are to be used, a circle of the same 

 size as the dish can be drawn with Chinese white on a black 

 card, the circumference divided into equal arcs, and radii drawn. 

 The dish is then laid on the card, the number of colonies in a 

 few of the sectors counted, and an average struck as before. 

 In counting colonies it is always best to aid the eye with a small 

 hand-lens. 



The Bacteriological Examination of the Blood. (a) This 

 may be done by taking a small drop from the skin surface, e.g. 

 the lobe of the ear. The part should be thoroughly washed 

 with i-iooo corrosive sublimate and dried with sterile cotton 

 wool. It is then washed with absolute alcohol to remove the 

 antiseptic, drying being allowed to take place by evaporation. 

 A prick is then made with a sterile surgical needle ; the drop 

 of blood is caught with a sterile platinum loop and smeared on 

 the surface of agar or blood serum. Film preparations for 

 microscopic examination may be made at the same time. It is 

 rare to obtain growths from the blood of the human subject 



