One of the most remarkable evidences of the adaptability of the yeasts to their surroundings and a specific characteristic occurs in what is sometimes called _ascospore_ formation. If a yeast cell finds itself lacking nourishment or in an unfavourable medium, it reproduces itself not by budding, but by forming spores out of its own intrinsic substance, and within its own capsule. To obtain this kind of spore formation Hansen used some gypsum blocks as medium on which to grow his yeast cells. Well-baked plaster of Paris is mixed with distilled water, and made into a liquid paste. Small moulds are made by pouring this paste into cardboard dishes, where it hardens again. The mould is sterilised by heat, and a small portion of yeast is placed on its upper surface, and then the whole is floated in a small vessel of water and covered with a bell-jar. Under these conditions of limited pabulum the cell undergoes the following changes: it increases in size, loses much of its granularity, and becomes homogeneous, and about thirty hours after being sown on the gypsum there appear several refractile cells inside the parent cell. These are the ascospores. In addition to the gypsum, it is necessary to have a plentiful supply of oxygen, some moisture (gained from the vessel of water in which the gypsum floats), a certain temperature, and a young condition of the protoplasm of the parent yeast cells. Hansen found that the lowest temperature at which these ascospores were produced was .5-3° C., and at the other extreme up to 37° C., which is blood-heat. The rapidity of formation also varies with the temperature, the favourable degree of warmth being about 22-25° C. [Illustration: GYPSUM BLOCK] Hansen pointed out that it was possible by means of sporulation to differentiate species of yeasts. For it happens that different species show slight differences in spore formation, _e. g._: (_a_) The spores of _Saccharomyces cerevisiæ_ expand during the first stage of germination, and produce partition walls, making a compound cell with several chambers. Budding can occur at any point on the surface of the swollen spores. To this group belong _S. pastorianus_ and _S. ellipsoideus_. (_b_) The spores of _Saccharomyces Ludwigii_ fuse in the first stage, and afterwards grow out into a promycelium, which produces yeast cells. (_c_) The spores of _Saccharomyces anomalus_ are different in shape from the others in that they possess a projecting rim round the base.