Page:The yeasts (1920).djvu/32

 Most of the Torula are easily recognized by their almost perfectly round shape, their content of fat, their peculiar manner of propagation which causes them to give off simultaneously many small round buds, and, finally, by their membrane, almost always surrounded by a layer of a mucilaginous substance. The genus Torulaspora and a few other yeasts have this same shape, which is known as the Torula type.

The Mycoderma and certain members of the genus Pichia often possess a decidedly refractive appearance, and elongated cylindrical cells which bud almost exclusively at two poles; their contents is transparent, enclosing a number of refractive granules localized especially in the extremities. This is the mycoderma type of yeast cell.

With S. Ludwigii, the cells possess a very peculiar form, tubuliform, bottle, or sausage shaped. Their division is intermediary between budding and partition.

Finally there are the Schizosaccharomyces, which are not ordinarily confused with other yeasts, for as the name indicates, they always multiply by division and not by budding. In ''Sch. octosporus'' the cells vary from a spherical form to the form of a drum stick. The spherical cells resemble huge micrococci while the drum-stick-shaped cells resemble the bacilli.

It is seen, then, that the yeasts present very common forms which are exhibited rather regularly by the various species. It is well to add that certain yeasts are able to assume abnormal forms. Thus, Lindner showed that S. Bailii, when growing in giant cultures on gelatin, resembled ameboid bodies.

It has been stated before that the yeasts may grow in a filamentous or mycelial formation. Nevertheless it does not occur in all of the yeasts, and never appears where there is feeble development. It appears, however, only under special conditions.

Mycelial formation was observed for the first time by Hansen in the growth which covered the surface of fermenting liquids; this is termed a pellicle or scum. This growth presents a very different appearance from that which is found upon the bottom of flasks. Colonies are composed of long threads and cells and, little by little, the growth takes on a resemblance of a mycelium. The formations, however, always remain in a rudimentary state.

The investigations of Hansen, Lindner, and Will have shown that certain yeasts are equally capable of forming the mycelial-like structure when growing on gelatin. It manifests itself very well in S. marxianus and carlsbergensis, Pichia membranaefaciens, in Zygo-