of that material, but the characters attributed to protoplasm cannot be detected in the minute glistening mass which makes up their body.

They are so certain to appear in a couple of days in organic infusions, or in any fluid prone to putrefaction, and multiply with such astounding rapidity, that they have been supposed by some to develop spontaneously. But this is now known not to be a fact. Bacteria can no more than any other form of living thing appear without progenitors. They float inanimate and dry in multitudes through our atmosphere, and adhere to all substances to which the air has free access. The moment they alight upon a suitable habitat, they burst into prodigious activity, at first forming masses or colonies, which may be seen as a jelly-like scum on the fluid. Such a habitat is supplied by any organic substance capable of ready decomposition, for which process, as is well known, the great requirements for life, moisture and warmth are to a certain degree necessary. Vast varieties of these organisms are now known. They differ slightly in shape, in their habitat, and in their properties. Some are obviously composed of two distinct layers, some are provided with a fine hair-like process, by the lash-like motions of which they move rapidly in a definite direction.

They are known to be inseparable from putrefactive changes in organic materials; without them no putrefaction can go on, since this process is but the product of their living activity. Great heat kills them, too great cold or dryness checks their activity and stops putrefaction. When an organic substance is absolutely protected from their presence by exclusion of the air, etc., no putrefaction occurs, even though it be prone to spontaneous decomposition, and be placed under favorable circumstances as to warmth and moisture.

Bacteria would not deserve so much notice here were it not for the pathogenic influence some of them have on the higher forms of life. We do not know that they are necessary for any of the more important processes that normally go on in the human body, though they are constantly present in the intestinal tract, and are inseparable from at least one change taking place

there that may be regarded as physiological.

It is their relation to the diseased state that makes a knowledge of these creatures imperative to medical men.

So long as the tissue of a higher animal is healthy and well nourished, the commoner forms of septic bacteria cannot thrive in immediate contact with it. They can only exist in the intestine, etc., because there they find accumulations of lifeless fluids which offer them a suitable nidus. Active living tissues may be said to have antiseptic power, i. e., are able to destroy septic bacteria; and it is only owing to this bactericide power of our textures, that we can with immunity breathe into our lungs the atmospheric air often crowded with these organisms, and swallow multitudes of them with our food. But for it every wound would become putrid, every breath might admit deadly germs to our blood.

When the vitality of the body generally is lowered, the vital activity of the tissue may fall below that necessary to insure the death of the bacteria, whose victory is signaled by unwonted and often fatal changes. Morbid fluids allowed to accumulate in the textures facilitate the growth of bacteria, and give rise to various grades of "wound infection." But if all accumulations be avoided, the bacteria brought into relation with the living tissue only irritate it, and cause general fever and local suffering to the patient. They cannot propagate in live tissue as in lifeless fluids. As a rule, the injurious effect of bacteria is in inverse proportion to the vital power of the textures which they invade. This is seen in many cases familiar to the physician and the surgeon. There are, however, many forms of pathogenic bacteria which, if introduced into the system by inoculation, are able to overcome the vital activity of the tissues of certain animals even in the most robust health.

We next come to forms of fungus, which set up a process very like putrefaction, such as the yeast plant, Torula cerevisia, which causes alcoholic fermentation in sugar solutions. In the torula an external case containing protoplasm may readily be seen, and multiplication of the cells goes on rapidly by a process of budding. Torule, however, like bacteria, though called vege

tables, have not the power of assimilating as ordinary green plants do, but require nutriment to be supplied to them which already contains organic or complex compounds. Structurally but little different from torula is a one-celled plant, the green protococcus, which, like a higher plant, can build up its texture from the simplest food stuffs, and carry on its functions. It consists of a case made of cellulose, within which lies a mass of protoplasm with a nucleus. Their protoplasm is colored green by a peculiar substance called chlorophyll. We shall see presently that it is to protoplasm containing chlorophyll that plants owe all their most characteristic and wonderful properties; viz., the property of assimilating so as to construct complex carbon compounds out of simple inorganic materials.

Two different forms of Amoeba in different phases of movement. Those on the left (after Cadiat.) A and B show an outer clear zone. (Gegenbauer.)

The smallest and simplest organisms classed as animals are generally larger than the vegetable cells just alluded to. They consist of protoplasm without any nucleus, and only sometimes with a structural difference between any part of their substance. As an example we may take Protamaba. This is a small mass of protoplasm without any nucleus, but its outer layer is clearer and less granular than the central part. It can move by sending out protoplasmic processes, in which currents can be observed resembling those in the vegetable cells. Excepting as regards the nucleus, it is much the same as the Amaba, which can be readily found and watched, and will be more accurately described.

The amoeba is a single cell or mass of uncovered protoplasm,

containing a well-defined nucleus, within which is a nucleolus. There is also generally a vacuole. The central part of the protoplasm is densely packed with coarse granules, but the outer, more active part is structureless and translucent looking, somewhat like a fine border of muffed glass, encasing the coarsely granular middle portion. Such an animal has no parts differentiated for special purposes, the requirements of its functions being so limited that the protoplasm itself can accomplish them.

Thus the processes of protoplasm, which flow out with considerable rapidity from the body, frequently encircle particles of nutrient material, and then closing in around them, press them into the midst of the granular central mass. Here they sojourn some time, and during this period no doubt any nutritive properties they possess are extracted from them, and they are then ejected from the plastic substance. This form of assimilation demands no previous preparation of the food such as we shall see takes place in the alimentary tract of man, and in the special organs of the higher animals; yet it is a form of digestion adequate at least to the requirements of this simple organism. The repeated alteration of relationship between the different parts of the protoplasm, and the surrounding medium during the flowing hither and thither of the currents, produces not only a change in the shape and position of the animal, but also acts as a means of distributing the nutriment to the different parts of the body, and of collecting and carrying to the surface the various products of tissue decomposition; thus the streaming protoplasm does the work of a circulating fluid such as we see in the more elaborate organisms for the distribution of nutriment and elimination of waste materials. The surface of the amoeba is sufficient to allow of the gas interchange necessary for life, and by means of the ever-changing material exposed, sufficent oxygen is taken for its tissue combustions, and so a function of respiration is established. The growth that results from the perfect performance of these vegetative functions proceeds until the maximum size is attained, and further nutritive activity is then devoted to reproduction. When growth ceases, commonly the cell divides and forms two distinct individuals. The movements which form

the most striking operations of the amoeba are the same as those which take place in protoplasm, except that they are more rapid and obvious. The clear, outer layer first flows out as a bud-like process, and, as it is gradually enlarging, some of the central granular part of the cell suddenly tumbles into its midst, where it remains, while other pseudopodia are being thrown out in the neighborhood, and the same changes repeated in them. It is difficult to watch the motions of an amoeba without being impressed with the idea that it is not only endowed with sensibility, but that it can also discriminate between different objects, for we see it greedily flowing around some food material, while it carefully avoids other substances with which it comes in contact.

If a glass vessel containing several amœbæ be placed in a window, they will be found to cluster on the side of the glass most exposed to the light. From this it would appear that, in some obscure way, protoplasm can appreciate light, and respond to its influence by moving toward it.

This single-celled animal, or nucleated mass of protoplasm, can perform all the functions of a higher animal. It can move from place to place and assimilate nutriment, apparently discriminating between different materials. It distributes nutrient stuffs and oxygen throughout its body by a kind of tissue circulation, and it can appreciate and respond to the most delicate form of stimulus, namely, light, which subtle motion has no effect on the sensory nerve fibres of the higher animals.

FIG. 41.

d

Diagram of Paradigestive cavity. (aa) Body space filled with soft

mæcium showing

protoplasm, into taken. (6) Mouth.

which food is

In some unicellular animals certain parts of the cell are specially modified for the performance of special functions, a division of labor thus taking place which insures the more perfect accomplishment of the different kinds of activity. In one of the commonest of the Infusoria (Paramacia bursaria), which swarm in dirty water, this is well exemplified. The outer layer of the flattened body is denser, and forms a kind of fibrillated corticular case (ectosarc),

(c) Anus. (d) Contractile vesi

cle. (After Lach

mann.)