176 BIOLOGY AND TECHNIQUE 



second tube contains the bacteria in much greater dilution than the 

 first and the colonies which will form in the plate poured from this 

 tube, will be farther apart. A third dilution is then made by trans- 

 ferring five loopfuls of the mixture in the second tube to the third. 

 This again is mixed as before. The contents of the tubes are then 

 poured into three sterile Petri dishes. The pouring should be done 

 with great care. The cover of the dish is raised along one margin 

 simply far enough to permit the insertion of the end of the test 

 tube, the plug of which has been removed and the lips passed, with 

 a rotary movement, through the flame. The medium is poured into 

 the dish without the lips of the tube being allowed to touch either 

 the bottom or the cover of the dish. The cover is then replaced 

 and the medium allowed to harden. 



FIG. 14. POURING INOCULATING MEDIUM FROM PETRI DISH. 



When agar has been used, the dishes may be placed in an in- 

 cubator at 37 C. It is well to place the plates upside down in 

 the incubator. This prevents the condensation water, squeezed out 

 of the agar during hardening, from collecting on its surface, and 

 forming channels for the diffuse spreading of bacteria. The same 

 end may be attained by the use of Petri plates provided with porous 

 earthenware lids, as suggested by Hill. Simple inversion of the 

 plates, however, usually suffices. When gelatin has been used, the 

 plates are allowed to remain in a dark place at room temperature 

 or in a special thermostat kept at 22 -25 C. 



Colonies in agar, kept at 37.5 C., usually develop in eighteen 

 to twenty-four hours ; those in gelatin or agar at room temperature 

 in from twenty-four to forty-eight hours, depending upon the species 



