GERMINATION STUDIES 



59 



possible. It is sometimes desirable to distribute the spores pre- 

 viously in a drop or small quantity of water, otherwise one may 

 get too many in the culture drop. The covers are then inverted 

 over the glass ring and pressed down so as to leave only one 

 minute unsealed area. 



It is an unwise and an inaccurate plan to use in the bottom of 

 the cell any other liquid than that used in the culture drop. This 

 must be so in order that 

 there may be no differ- 

 ences of vapor pressure, 

 and consequently no evap- 

 oration from drop to'liquid 

 below, or vice versa. For 

 instance, it would be man- 

 ifestly absurd to test ger- 

 mination in a drop of, say, Fig. 13. Rings for Drop Cultures 



S per cent alcohol above if "< cemented to slide ; i, in Petri dish with filter paper 



there were only pure water below. If there is danger of contami- 

 nation below, and consequent interception of the light, thorough 

 sterilization must be given beforehand. If the drop cultures are 

 made as soon as the slides are prepared, sterilization should not 

 be necessary, since all parts are flamed. Sterilization may be 

 given at any time, however, previous to the ringing with vase- 

 line. It is usually sufficient to sterilize the cells in a dry oven at 

 a temperature of from 110 to 115 C. This temperature melts 

 the wax, but if the slides are level, there is no danger that the 

 cells will slip. A temperature much higher is not to be recom- 

 mended. Another convenient method of sterilization is by means 

 of formalin. The cells are filled with a solution of from 3 to 5 

 per cent formalin, and this is allowed to stand for half an hour ; 

 then on being rinsed with distilled water, again filled with the 

 water, and left for ten minutes they will be found sterile. The 

 cells should then be inverted and dried before the ringing with 

 vaseline is effected. By this process some cells will become dis- 

 connected ; but if the cementing has been well done, this is a 

 matter of small importance. 



Since it will often be found desirable to invert the slides and 

 cells to prevent contamination while awaiting use, the work should be 



