applied to the new facts would soon solve the riddle of life by revealing the minutiae of structure on which functions depended. A vision of a grand simplicity of organic nature dawned upon their minds. It seemed probable that all living beings were composed of very minute organic units built up into a number of gross organs, the functions of which depended on the arrangement of their constituent units. This was the cell-theory of Schleiden and Schwann, who established the general fact that both plants and animals are built up of minute cells, and that the first starting point of both plant and animal is a nucleated cell. This splendid generalisation is without doubt the greatest discovery as yet made with the microscope.

Plate I, Fig. 2, may serve as the plan of such an elementary cell, greatly magnified. a is the outer skin, or cell-membrane; b, the living matter, or protoplasm, having in its centre c the "nucleus," in which the life of the cell and the power of multiplying itself chiefly reside.

Plate I, Fig. 3, represents a vertical section of a portion from the leaf of a plant, magnified 350 times. At the lower edge one of the stomata or pores is seen.

Plate I, Fig. 4, is from the mucous membrane of the human mouth, magnified 1,000 times.

These examples may serve as proofs that both vegetables and animals are built up of cells; and since the highest powers of our microscopes reveal no more, the cell theory seems to bring us very near the innermost mysteries of life.

To the cell-theory we owe our conception of the organism as a body composed of protoplasm, the real living matter-and of formed material, the non-living or semiliving framework. The former is the true seat of life, and the latter is produced as a result of its vital activity. The

founders of this theory first showed us that the living structure is usually arranged in, and works through, small structural units called cells, and that the organism is composed of independent or semi-independent individuals. associated together in a colony for the common good.

It was observed that the simplest vegetable and animal cells were filled with a substance nearly transparent and colourless, semi-fluid, much resembling the raw white of an egg, which was supposed to be living matter in its earliest, most elementary form, to which the term Protoplasm has been applied. Some, as Professor Huxley, have imagined that the protoplasm is the same, whether found in plants or animals, of which we have no proof whatever; and it is otherwise very unlikely. There may be many forms of this elementary living matter, but we cannot analyse it, for it dies at the moment our analysis begins. The most recent researches go to show that the substances taken as food undergo many changes before they become part and parcel of the living body. The matter of a meal becomes reduced almost to its elements before it is converted into living flesh. It would seem as if the qualities of each particle of living matter were of such an individual character that it had to be built up afresh from the very beginning hence the construction which is continually going on as well in the animal as in the vegetable body. This constructive process is an upward series of changes, which may be compared to a stair of many steps, by which the dead food, of varying simplicity or complexity, is, by the vital energy, built up into more and more complex bodies. The summit of this stair is the protoplasm, or living matter. Then commences a downward series of many steps or changes, in which these more complex bodies are broken down with the setting free of energy into simpler and simpler waste products. This is but a

mere glimpse at the complex process by which living matter has the power of transforming the dead food into its living self,-of creating life from the dead.

But let me again remind you that nothing that lives is alive in every part. No one would maintain that the shell of an oyster, or a mussel, was, like the moving shell-fish itself, in a living state. Nevertheless, the shell grows, but that growth is restricted to certain points. It grows at the free edge, and on the inner surface, and thus increases its dimensions. By far the larger part of the shell is as lifeless, while it yet remains connected with the living animal, as it is when empty of its occupant and maker. The new matter which is added to it by the living creature, is prepared and formed by the living matter alone. In man and the higher animals, the free portions of the nails and hair, the outer skin, and much of the teeth, are evidently lifeless. But the waste and removal of these is compensated for by the addition of new matter by living particles. Of the internal tissues, a great part, especially the fatty and cellular tissues, is also in a nonliving state, though they perform important services of a passive kind, and are connected with matter actually alive.

One of the most elementary forms of life that we can study is that common animalcule found adhering to weeds and bits of dead wood in stagnant water, known to the naturalists as the Amoeba. It is scarcely visible to the naked eye, but when magnified appears as a shapeless tiny blob of jelly, nearly colourless. Under a higher magnifying power it appears to consist of a granular substance, enclosed in a fine colourless envelope. The Amoeba is never the same shape for long together. Leave it for half-an-hour and look at it again, it has altered so much as not to look like the same thing. Portions are pushed out from the main body to form