To begin with, you must have a spoonful or two of yeast to look at while I talk. You will probably notice first a number of bubbles, like soap-bubbles; but, instead of common air, the yeast-bubbles contain a gas made by the yeast—carbonic-acid gas. Next you will notice the brownish color of the yeast; it grows thicker and muddy, and after an hour or so begins to rise. This rising the chemists call fermentation; biologists call it growing. The spoonful has become a cupful. The yeast is really alive, and it is one of the simplest forms of life. In studying biology, then, or the science of life, we begin at what seems the beginning.
All that I have described you can see with your own eyes; but now I must tell you something about the yeast which you could never find out with your eyes alone. With the aid of the microscope, a great many little solid bodies are seen floating about in it. Sometimes they are found alone, but most frequently in groups (Figs. 1-5),
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| Fig. 1.—Round Cells. | Fig. 2.—Lemon-shaped Cells. | Fig. 3.—Outer and Inner Surface of Sac. |
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| Fig. 4.—Group of Cells. | Fig. 5.—Group of Cells. |
and each one is about 13000 of an inch in size. Though they are solid, yet we can see through them, and they are always round (Fig. 1), some of them not quite as round as a ball, more like a lemon (Fig. 2), but none of them are square or flat. The cover of each one is double (Fig. 3), that is, it has an outside and an inside. Under the microscope, these two surfaces look like two round lines, one within the other (Fig. 6). Inside these lines is something which looks like little grains (Fig. 7); and this whole cover, with all that is inside of
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| Fig. 6. | Fig. 7. | Fig. 8. | Fig. 9.—Yeast-cell, or Torula. a, Jelly, or Protoplasm; |
it, is called a cell. Now you must learn of what these cells are made. First there is the outside part, which is like a bag or sac. This bag is tough and solid, and is full of a jelly-like substance, which is thick
