Page:A Text-book of Animal Physiology.djvu/40

 The yeast-cell is now believed to possess a nucleus.

Chemical.—When yeast is burned and the ashes analyzed, they are found to consist chiefly of salts of potassium, calcium, and magnesium.

The elements of which yeast is composed are C, H, O, N, S, P, K, Mg, and Ca; but chiefly the first four.

Physiological.—If a little of the powder obtained by drying yeast at a temperature below blood-heat be added to a solution of sugar, and the latter be kept warm, bubbles of carbon dioxide will be evolved, causing the mixture to become frothy; and the fluid will acquire an alcoholic character (fermentation).

If the mixture be raised to the boiling-point, the process described at once ceases.

It may be further noticed that in the fermenting saccharine solution there is a gradual increase of turbidity. All of these changes go on perfectly well in the total absence of sunlight.

Yeast-cells are found to grow and reproduce abundantly in an artificial food solution consisting of a dilute solution of certain salts, together with sugar.

Conclusions.—What are the conclusions which may be legitimately drawn from the above facts?

That the essential part of yeast consists of cells of about the size of mammalian blood-corpuscles, but with a limiting wall of a substance different from the inclosed contents, which latter is composed chiefly of that substance common to all living things—protoplasm; that like other cells they reproduce their kind, and in this instance by two methods: gemmation giving rise to the bead-like aggregations alluded to above; and internal division of the protoplasm (endogenous division).

From the circumstances under which growth and reproduction take place, it will be seen that the original protoplasm of the cells may increase its bulk or grow when supplied with suitable food, which is not, as will be learned later, the same in all respects as that on which green plants thrive; and that this may occur in darkness. But it is to be especially noted that the protoplasm resulting from the action of the living cells is wholly different from any of the substances used as food. This power to construct protoplasm from inanimate and unorganized materials, reproduction, and fermentation are all properties characteristic of living organisms alone.

It will be further observed that these changes all take place within narrow limits of temperature; or, to put the matter