CULTIVATION AND SYSTEMATIC STUDY OF BACTERIA. 153 



ferred to three sterilized Petri dishes. After numbering and label- 

 ing, the dishes are set aside for future examination. If colonies oc- 

 cur on plates two and three, they are probably pure cultures of the 

 species desired. From these colonies tubes containing media can 

 be inoculated. 



HANGING-DROP CULTURES. 



It is often very desirable to make a microscopic study of bacteria 

 and other organisms in the process of growth. This is accom- 

 plished by resorting to the hanging drop culture. A small drop of 

 the liquid media containing the species to be studied is transferred 

 by a platinum loop to the center of a sterilized cover-glass. This is 

 then inverted upon a slide in the center of which a concavity has 

 been ground out. The edges of the cover-glass are then sealed with 

 a layer of vaseline applied with a camelVhair brush. Th<3 prepara- 

 tion may then be studied from time to time, the focusing being ap- 

 plied to the edges of the preparation, rather than the center, which 

 will generally be found opaque. 



INOCULATION OF ANIMALS. 



To save human life it is of 1 en quite necessary to sacrifice the lives 

 of lower animals. The experiments of bacteriologists, while ap- 

 pearing to the superficial observer as almost merciless, are in the 

 interests of the highest humanity and are destined not only to di- 

 minish in a large degree the sum of human suffering, but to bring 

 alleviation to lower animals, at whose expense the requisite knowl- 

 edge is sought. The life of man outweighs that of a mouse or 

 " many sparrows.*' For purposes of experimentation, such animals 

 as mice, rats, guinea pigs, and rabbits are generally employed. In- 

 oculations are made in the ear, at the root of the tail, and elsewhere. 

 The hair is first removed by cutting or searing. A V-shaped inci- 

 sion is then made, and the infected material inserted by means of a 

 platinum wire. Inoculation of the blood may be made with a bac- 

 teriological syringe. 



SYSTEMATIC STUDY OF BACTERIA. 



The identification of any species can only be made after a thor- 

 ough study of its characteristics. Even then the determination will 



