GLYCERIN AGAR-AGAR. 331 



that one familiar with the appearance can anticipate 

 with tolerable certainty the results of microscopic exam- 

 ination. 



Glycerin Agar-agar. — Upon nutrient glycerin 

 agar-agar the coloni&s 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 sur- 

 face, as very flat, almost transparent, dry, non-glisten- 

 ing, round points which are not elevated above the 

 surface upon which they are growing. When slightly 

 magnified they are seen to be granular, and to present 

 an irregular central marking which is denser and darker 



Fig. 66. 



^ ^ -<^' A 



• ^ 0-- 



a o 



Colonies ot bacittus diphUterix on glycerin-€iS&r-agar. a. Colonies located 

 in the depths of the mediam. b. Colonies just breaking out upon the snrEace 

 of the medium, c. Fally developed surfece-colony. 



by transmitted 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 scalloped. (Fig. 66, c.) These colonies are always 

 quite dry in appearance. AYhen deep down in the agar- 

 agar they are coarsely granular. (Fig. 66, a.) They 

 rarelv exceed 3 mm. in diameter. 



