different forms, depending upon the medium in which they are cultivated and their age. It is thus, for example, in old cultures that the wider cells diminish generally at the expense of the longer ones. The different species of yeasts have somewhat the same shape and are distinguished with some difficulty from one another. If S. cerevisiae is compared with S. Pastorianus or S. ellipsoideus, quite noticeable differences are apparent. While S. cerevisiae usually presents round cells and S. ellipsoideus egg-shaped cells, S. Pastorianus presents, to the contrary, elongated cells, often in the shape of a sausage. But besides these elongated forms, one may find in cultures round cells which may scarcely be differentiated from S. cerevisiae or S. ellipsoideus. On the other hand, in culture of S. cerevisiae and S. ellipsoideus may be found round cells, and also elliptical cells which bear much resemblance to S. Pastorianus. It is thus apparent that these three species may not be closely differentiated by the shape of the cells. There is always a predominating form which attracts attention; with S. cerevisiae the predominating form is round ; with S. ellipsoideus it is elliptical, while with S. Pastorianus it is most frequently elongated.
The majority of the yeasts, notably those of industrial importance (beer, alcohol, wine and cider), present a mixture of spherical and elongated cells. Although this is the case, a predominating form exists which may be of three types, the cerevisiae type, the ellipsoideus type or the Pastorianus type.
Among the yeasts, which are very numerous and in which the cell shapes are variable and indefinite, are often found certain species, or groups of species, in which the cells present a characteristic shape and which are separated closely from the preceding yeasts. Hansenia apiculata, for example, offers cells which are usually of the shape of a lemon, being provided with small projections from which the name apiculata is derived. A series of species of yeasts is known which possesses a similar shape and these, without doubt, are varieties of Hansenia apiculata or neighboring species. (See Fig. 4, g.)
