CULTIVATION BY STROKE AND PUNCTURE. 69 



when the colonies are in the centre. These plate 

 cultivations on slides must of course be preserved in 

 sterilised cultivation dishes. 



Quite the most convenient way to observe the de- 

 velopment of bacteria with the microscope is to use 

 the hanging drop cultivations already described. A- 

 number of slides with concave depressions are sterilised 

 in the hot-air apparatus, together with a number of 

 larger cover- glasses, after which they are placed with 

 strong sterilised forceps in sterilised cultivation dishes. 

 A cover-glass is then placed near to each depression, 

 and the lids are put on immediately. A small drop of 

 some fluid medium — gelatine, agar-agar, or bouillon — 

 is taken out of the test-tube with a sterilised pipette 

 and placed in the middle of each cover-glass. When 

 the gelatine or agar-agar has solidified, the smallest 

 possible quantity of the material for examination, so 

 minute as to be imperceptible to the naked eye, is 

 introduced into the middle of the drop, and the cover- 

 glass is turned over and placed over the depression in 

 the slide, so that the solidified drop hangs freely in 

 the cavity. The edges of the cover-glass are then 

 smeared over with vaseline, partly to keep it safely 

 on the slide, and partly to shut out the air. This 

 however is hardly necessary if the slide is replaced in 

 the cultivation dish, in which case it is best only to put 

 vaseline on one corner of the cover-glass. The opening 

 of the pipette must be very small, so that the drops 



