624 LABORATORY HINTS 



work more quickly as the agar soon solidifies and the organisms are 

 not evenly distributed. When the agar has set invert dishes to 

 prevent water of condensation spoiling colonies. 



" Streak " Cultures. 



An oblique slope of agar or nutrient gelatin in a test-tube is taken, 

 and with a sterilized platinum loop a trace from the culture is drawn 

 up the slope. 



"Stab" Cultures. 



A test-tube of solid media is taken, and the straight platinum wire 

 with the growth on the tip is introduced down the centre of the medium. 



" Shake " Cultures. 



Liquefy the gelatin in a test-tube by putting it in a beaker of water 

 at 40° C, and inoculate the medium with the organism. Gently mix 

 to distribute the organisms; gas producing organisms form bubbles. 



"Anaerobic" Cultures. 



Glucose agar or gelatin is most frequently used. A tube three- 

 quarters full is kept in boiling water for five minutes to soften the 

 medium and expel the oxygen. After the stab is made, warm the top 

 of the medium to seal the needle track. For fluid media place thereon 

 a layer of olive oil, and then inoculate with a sterile pipette. 



In diflFerentiating growth note: — 



If the colonies are separate, rounded, minute or large indefinable 

 masses. Plague bacteria grows in small rounded masses, but Coli 

 form one creamy mass. Notice if any colour is produced and if the 

 medium is stained as with Bacillus pyocyaneus. 



Does the growth spread aw^ay from the line of inoculation, and if 

 so, what kind of an edge does it form ? Is gelatin liquefied, broth 

 rendered turbid? Does a scum form on the surface? Is the deposit at 

 the bottom ? Does it produce acid or gas or both in a sugar medium ? 

 Is milk clotted or acid formed in it? Will it grow on potato, and if 

 so, is any colour produced ? 



THE PREPARATION OF SECTIONS. 

 (1) Fixing the Tissues. 



Place the piece of tissue in 10 per cent, formalin, twelve to 



twenty-four hours. 

 Wash in running water and then proceed to harden it. 

 After it is hardened the tissue must be prepared for section 



cutting. 

 This is by freezing or embedding in paraffin. 



