METHODS OF CULTURE AND OF EXAMINATION. 95 



rubbed successively with the inoculated platinum needle, or 

 similar body {fractional streak). 



The finished plates are kept in culture-boxes for from two 

 to four days at room-temperature, or in the thermostat 

 at a temperature of 24 C. (75.2 F.) or 25 C. (77 F.) 

 (gelatin-plates), or they are permitted to remain in the 

 thermostat for from twenty -four to forty-eight hours at a 

 temperature of 37 C. (98.6 F.) (agar-plates). After the 

 lapse of this time the plates are examined with the naked 

 eye, with a lens, and also with- a low power of the micro- 

 scope (from 50 to 100 magnifications). It is next deter- 

 mined whether one or several varieties of colonies have 

 developed, and then the peculiarities of these colonies 

 whether they liquefy the gelatin, whether they possess a 

 sharp or an irregular border, w r hether they are granular, 

 whether they present a definite arrangement,, special tints 

 of color, etc. Variations in the appearance of similar col- 

 onies are dependent upon their position in the gelatin. 

 Deeply lying colonies almost always assume a spheric 

 shape, and appear, as a rule, round and dark, whereas the 

 superficial colonies sometimes spread like a membrane upon 

 the surface of the gelatin, and appear bright and transparent. 

 The most important object to be accomplished, however, is 

 to obtain pure cultures from these plates. If the particular 

 colony from which it is intended to make inoculations is 

 not entirely too small, and if it be isolated, the procedure is 

 quite simple : The plate or the dish is placed upon a dark 

 background, the platinum wire is heated in the flame, and 

 with the aid of the naked eye or of a lens the extremity 

 of the wire is introduced into the colony. If, however, the 

 colony is particularly small, there is no alternative but to 

 make the inoculation with the platinum wire with the aid of a 

 low power of the microscope. For this purpose a steady hand 

 and much practice are required. Apparatus have also been 

 devised for this purpose, but they are rather complicated, and 

 scarcely more trustworthy for the experienced manipulator. 

 After the material from the colony has been taken up with 

 the needle, it is smeared upon one of the various culture- 

 media after having convinced oneself, with the aid of the 

 microscope, if necessary, that only one colony has been re- 

 moved, and in this way a pure culture is made. In inoc- 

 ulating a gelatin-tube the needle is generally introduced 

 from above downward through the middle of the column of 



