These may be incubated either ~aerobically~ (i. e., in the presence of oxygen) or ~anaerobically~ (i. e., in the absence of oxygen, or in the presence of an indifferent gas, such as hydrogen, nitrogen, or carbon dioxide). With regard to the temperature at which the cultivations are grown, it may be stated as a general rule that all media rendered solid by the addition of gelatine are incubated at 20°C., or at any rate at a temperature not exceeding 22° C. (that is, in the "cold" incubator); whilst fluid media and all other solid media are incubated at 37° C. (that is, in the "hot" incubator). Exceptions to this rule are numerous. For instance, in studying the growth of the psychrophylic bacteria, the yeasts and the moulds, the cold incubator is employed for all media. Tube cultivations are usually packed in the incubator in small tin cylinders, such as those in which American cigarettes are sold, or in square tin boxes. Beakers or tumblers may be used for the same purpose, but being fragile are not so convenient. Metal test-tube racks, long enough to just fit into the interior of the incubator and each accommodating two rows of tubes, are also exceedingly useful. ~AEROBIC.~ ~The Preparation of Tube Cultivations.~ The preparation of a tube cultivation consists in: (a) Inoculating a tube of sterile nutrient medium with a portion of the material to be examined. (b) Incubating the inoculated tube at a suitable temperature. The details of the first of these processes must be varied somewhat according to whether the tubes of nutrient media are inoculated or "planted" from-- 1. Pre-existing cultivations. 2. Morbid material previously collected (_vide_ page 373). 3. Fluids, tissues, etc., or from the animal body direct. The method of preparing tube cultivations from pre-existing cultivations is as follows: [Illustration: FIG. 117.--Inoculating tubes, seen from the front.] ~1. Fluid Media~ (e. g., Nutrient Bouillon).-- 1. Flame the cotton-wool plug of the tube containing the cultivation and also that of the tube of sterile bouillon. 2. Hold the two tubes, side by side, between the left thumb and the first and third fingers, allowing the sealed ends to rest on the dorsum of the hand, and separating the mouths of the tubes (which are pointed to the right) by the tip of the second finger. Keep the tubes as nearly horizontal as is possible without allowing the fluid in the bouillon tube to reach the cotton-wool plug (Fig. 117). 3. Sterilise the platinum loop and allow it to cool.[8] 4. Grasp the plug of the tube containing the cultivation between the little finger and palm of the hand and remove it from the tube. 5. Grasp the plug of the bouillon tube between the fourth finger and the ball of the thumb and remove it from the tube.