CULTURES, AND THEIR STUDY. 21 1 



agar tube, without touching the surface. The tube is 

 then inclined so that the water flows over the agar, 

 after which it is stood away in the vertical position. 

 Colonies will grow where bacteria have been floated upon 

 the agar-agar, and may be picked up later in the same 

 manner as from a plate. 



In other cases pure cultures may best be secured by 

 animal inoculation. For example, when the tubercle 

 bacillus is to be isolated from milk or urine which con- 

 tains rapidly growing bacteria that would outgrow the 

 slow-developing tubercle bacillus, it is better to inject 

 some of the fluid into the abdominal cavity of a guinea- 

 pig and await the development of tuberculosis, and then 

 seek to secure the bacillus from the unmixed material in 

 the softened lymphatic glands. Anthrax bacilli are also 

 more easily secured in pure culture by inoculating a 

 mouse and recovering the bacilli from a spleen or the 

 heart's blood after death, than by going to the trouble of 

 making plates and picking out the colonies. 



In many cases when it is desired to isolate the micro- 

 coccus tetragenus, the pneumococcus, and others, it is 

 easier to inoculate the most susceptible animal and 

 recover the germ from the organs than to plate it out and 

 search for the colony among many others which may be 

 similar to it. 



The development of bacteria in liquids is of less in- 

 terest than that upon solid media. The growth generally 

 manifests itself by a diffused turbidity. Sometimes flocculi 

 float in the otherwise clear medium. Some forms grow 

 most rapidly at the surface of the liquid, and produce a 

 distinct membranous pellicle called a mycoderma. In 

 such a growth multitudes of degenerated bacteria and 

 large numbers of spores are to be observed. On the 

 other hand, it occasionally happens that the growth 

 occurs chiefly below the surface, and may produce gelat- 

 inous masses which are known as zooglea. 



In gelatin the bacteria exhibit a great variety of ap- 

 pearances, many of which are beautiful and interesting. 



