148 PATHOGENIC BACTERIA. 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 zobglea. In gelatin the bacteria exhibit a great variety of ap- pearances, many of which are beautiful and interesting.