NOTE.--When gelatine is stored for some time the upper surface of the cylinder becomes concave owing to evaporation. Tubes showing this appearance should be liquefied and again allowed to set before use for stab culture, otherwise when the needle enters the medium, the surface tension will cause the gelatine cylinder to split. [Illustration: FIG. 118.--Sloped or slanted medium for streak or smear culture.] [Illustration: FIG. 119.--Straight tube.] _Shake Culture._-- 1. Liquefy a tube of nutrient gelatine (or agar, or other similar medium), by heating in a water-bath (Fig. 121). 2. Inoculate the liquefied medium and label it, etc., precisely as if dealing with a tube of bouillon. 3. Place the newly planted tube in the upright position (e. g., in a test-tube rack) and allow it to solidify. 4. Label the tube; when solid, incubate. _Esmarch's Roll Cultivation._-- 1. Liquefy three tubes of gelatine by heat. 2. Prepare three dilutions of the inoculum (as described for plate cultivations, page 228, steps 4 to 7). 3. Roll the tubes, held almost horizontally, in a groove made in a block of ice, until the gelatine has set in a thin film on the inner surface of tube (Fig. 120); or under the cold-water tap. [Illustration: FIG. 120. Esmarch's roll culture on block of ice.] In order that the medium may adhere firmly to the glass, the agar used for roll cultivation should have 1 per cent. gelatine or 1 per cent. gum arabic added to it before sterilisation. Roll cultivations, which served a most important purpose in the days before the introduction of Petri dishes for plate cultivations, are now obsolete in modern laboratories and are merely mentioned for the benefit of students, since examiners who are interested in the academic and historical aspects of bacteriology sometimes expect candidates to be acquainted with the method of preparing them. The Preparation of Plate Cultures. If a small number of bacteria are suspended in liquefied gelatine, agar, or other similar medium, and the infected medium spread out in an even layer over a flat surface and allowed to solidify, each individual micro-organism becomes fixed to a certain spot and its further development is restricted to the vicinity of this spot. After a variable interval the growth of this organism becomes visible to the naked eye as a "colony." This is the principle upon which the method of plate cultivation is based and its practice enables the bacteriologist to study the particular manner of development affected by each species of microbe when growing (a) unrestricted upon the surface of the medium, (b) in the depths of the medium. The method itself is as follows: ~Apparatus Required.~-- 1. Tripod levelling stand. 2. Large shallow glass dish, with a square sheet of plate glass to cover it. 3. Spirit level.