GELATIN AND POTATO MEDIA. 191 



When gelatin tubes, and other plugged glass-ware have 

 been kept for some time, more or less dust settles upon the 

 cotton. There is, therefore, always danger of contamina-_ 

 tion when removing a plug. This risk can be reduced to a 

 minimum, if the cotton is touched to a flame before it is with- 

 drawn. It is advisable, as a matter of routine, to scorch 

 every cotton plug before it is removed. 



Laboratory work. The student will make a careful macroscopic 

 and microscopic examination of the various plates. Stab cultures 

 are to be made of each organism studied. Hanging-drop examina- 

 tions and permanent stained preparations should be made, either from 

 the colonies or from the stab cultures. The operation of fishing- 

 should be practised until it can be done readily and successfully. It 

 will be well to have another person watch the operation, to call at- 

 tention whenever the wire touches either the lens or a part of the 

 plate, other than that intended. Should the wire touch anything but 

 the colony intended, it must be immediately sterilized before repeat- 

 ing the attempt. 



The line of study of the Bacillus prodigiosus, Bacillus 

 Indicus and of the various bacteria to be presently taken 

 up consists, first, in making plate cultures. Colonies are 

 thus obtained, the characteristics of which are thoroughly 

 studied. Hanging-drop examinations, and stained prepara- 

 tions are next made, in order to become familiar with the 

 organism itself. Stab cultures in gelatin are then made, 

 and also streak cultures on potato. Later on, when the 

 study of the pathogenic organisms is taken up, cultures 

 are also made on agar, in bouillon and in milk. Finally 

 drawings are made showing the form of the colony, the 

 form of the organism, the appearance of the stab culture, 

 etc. 



In order to economize on gelatin, one-half of the class 

 makes plates of one set of organisms, and, the next time r 

 the other half of the class does the plating. Each student, 

 however, is expected to examine and study every organism 

 which is given out. 



