GELATIN. 301 



are often the only colonies that attract attention, and if 

 others of similar size are present, they are generally of 

 quite a different aspect. Its colonies are large, round, 

 elevated, grayish-white, or yellowish with a centre more 

 opaque than the slightly irregular periphery. The sur- 

 face of the colony is at first moist, but after a day or two 

 becomes rather dry in appearance. 



A blood-serum tube studded over with coalescent or 

 scattered colonies of this organism is so characteristic 

 that one familiar with the appearance can anticipate 

 with tolerable certainty the results of microscopic ex- 

 amination. 



GLYCERIN-AGAR-AGAR. Upon nutrient glycerin- 

 agar-agar the colonies likewise present an appearance 

 that may readily be recognized. They are in every way 

 more delicate in their structure than when on the serum 

 mixture. They appear at first, when on the surface, as 

 very flat, almost transparent, dry, non-glistening, round 

 points which are not elevated above the surface upon 

 which they are growing. When slightly magnified 

 they are seen to be granular, and present an irregular 

 central marking which is denser and darker by trans- 

 mitted light than the thin, delicate zone which surrounds 

 it. As the colony increases in size the thin granular 

 peripheral zone becomes broader, is usually marked by 

 ridges or cracks, and its periphery is notched or scal- 

 loped. (Fig. 59, o.) These colonies are always quite 

 dry in appearance. When deep down in the agar-agar 

 they are coarsely granular. (Fig. 59, a.) They rarely 

 exceed 3 mm. in diameter. 



GELATIN. On gelatin the colonies develop much 

 more slowly than on the other media that can be 

 retained at a higher temperature. They rarely present 



14 



