physiologists seem to be nearer its solution than they were a quarter of a century ago. Both problems doubtless involve many of the same elements, so that a solution of the one would go far toward elucidating the other. The recent advances in the study of the chemistry of colloids have afforded stimulating suggestions as to how both of the fundamental life processes of transmission and contractility may be interpreted in terms of the reversible changes of colloidal solutions. To the extent that these interpretations are successful, to that extent we may be said to succeed in explaining life.

It seems almost ridiculous to call blood a tissue, but it is often so designated because it contains living cells. The fluid part, or plasma, is a very complex mixture of dissolved proteins, fats, carbohydrates, salts, waste products of metabolism, and many other substances in smaller amounts. In the vertebrate animals the corpuscles, or cells of the blood, are of two kinds, the red and the white. The former are circular or oval cells, whose cytoplasm contains a large amount of a peculiar pigment called hemoglobin, which gives to the blood its red color. Hemoglobin is an iron-containing protein which takes in oxygen when this gas is relatively abundant in the lungs, and gives it up with great readiness where the oxygen pressure is relatively low, as it is in the tissues of the body. It is admirably adapted, therefore, to serve its function as an oxygen carrier to the tissues. In mammals the red blood cells have no clearly defined nucleus, although they originated in nucleated cells, but in most of the lower vertebrates the red cells are furnished with a distinct nucleus.

FIG. 21 - Human blood corpuscles: r, the red cells; 1, a white cell.

The white blood cells, or leucocytes, are nucleated cells with naked protoplasm of irregular and changeable form. Their mode of behavior is closely similar to that of an Amœba, which will be described in a later chapter. We may think of our white corpuscles as really parts of us, but they appear to be little organisms quite as independent in their movements as if they

were parasites in our blood. They originate, however, from several organs of the body, particularly the lymph glands and the marrow of the bones. Unlike the red cells which are passively carried in the blood stream, the white cells have the power of active locomotion, and may even creep through the delicate walls of the capillaries and other thin membranes and pass out of the body entirely. Great numbers of them wander into the alimentary canal, and into the nasal cavity and mouth. These white cells engulf and devour materials of broken-down tissues, and they attack and destroy many kinds of bacteria which may have succeeded in invading the body. They are therefore important defenders of the organism, and they have the property of collecting in large numbers in regions of unnatural irritation and seats of infection to which they are attracted by some kind of chemical stimulus. In the colorless fluid called lymph there are many white cells, but no red ones.

In the blood of most animals below the vertebrates, there are, except in rare instances, no red cells. The corpuscles are usually of the colorless amoeboid type, more or less like the leucocytes of our own body. Hemoglobin is present in the blood of many kinds of worms and molluscs, but it is generally dissolved freely in the plasma. In most of the molluscs and crustaceans there is a related oxygen-carrying compound called hemocyanin, which often gives the blood a bluish tinge, and which contains copper instead of iron.

REFERENCES

BÖнм, A. A., AND VON DAVIDOFF, M. A., Text Book of Histology (2nd ed.). Philadelphia, Saunders, 1909.

DAHLGREN, U., AND KEPNER, W. A., Principles of Animal Histology. N. Y., Macmillan, 1908.

CHAPTER V

THE ONE-CELLED FORMS OF LIFE

A large proportion of the species of both plants and animals is composed of organisms consisting of a single cell. These forms are minute in size, and until less than three centuries ago, they remained completely hidden from the ken of man. The credit for revealing the existence of this world of minute life belongs, more than to any one else, to an industrious Dutch weaver and maker of lenses, Anton van Leeuwenhoek. Happening to examine some stagnant rain water, Leeuwenhoek saw to his surprise numerous small creatures swimming about in the most lively manner. These creatures were of the most diverse form and behavior, and Leeuwenhoek sent many accounts of his observations to the Royal Society of London, which published them in its Philosophical Transactions. Thus was started a series of discoveries which have opened up new fields of knowledge and have yielded practical results of inestimable value to the human

race.

A few years after the cell theory was set forth by Schleiden and Schwann, it came to be recognized that microscopic organisms constitute a very motley lot, and that many forms are multicellular in composition, whereas others are composed of only a single cell. The one-celled animal organisms are called the Protozoa, and the one-celled plants, the Protophyta; but among these primitive forms of life the plant and animal kingdoms draw more closely together as if converging toward a common root. In fact, there are many organisms which so combine plant and animal characteristics that it is impossible to decide, except arbitrarily, to which group they should be assigned.

Taking up first the one-celled animals, we may find it advantageous to consider, as a representative of a large group of the

Protozoa, the common Ameba proteus. This well-known organism, which is a frequent inhabitant of ponds and streams, is often described as a minute, irregular, jellylike mass of protoplasm, containing a nucleus. There is no permanent cell wall, and the outer protoplasm, or ectoplasm, is more transparent and of somewhat firmer consistency than the inner, more fluid and granular endoplasm. The shape of an Amoeba is subject to frequent changes due to the formation of lobes or projections called pseudopodia (literally false feet). When one watches a

FIG. 22-Amaba proteus: A, active state; B, in division; C, in form of cyst. cv, contractile vacuole; n, nucleus; p, pseudopod showing clear ectoplasm at the tip; 1, 2, 3, 4, stages in the formation of a food cup and in ingesting a particle of food, F.

moving Amoeba he may see, where a pseudopod is forming, that the ectoplasm appears to liquefy and then to give way, allowing the endoplasm to come to the surface. The latter then quickly forms a denser transparent layer where it comes in contact with the water. The granules of the protruding protoplasm are squeezed toward the center, thus causing the new pseudopod to become more transparent. Ectoplasm is formed at the expense of endoplasm and vice versa. The newly formed ectoplasm of a protruding pseudopod is adhesive, especially at the tip; this apparently forms a point of attachment to the solid surface on

which the Amoeba creeps, and the contractility of the ectoplasm pulls the organism along. Then another pseudopod may be formed on the same side, and, by acting in a similar manner, enables the organism to creep farther in the same direction.

Amoeboid movement has been explained in a variety of ways. One view is that it is caused by local variations in surface tension due to external and internal changes. Phenomena resembling amoeboid movement can be brought about in oil drops suspended in water if one brings in contact with the surface of the drop some chemical which reduces the surface tension at the boundary between the water and the oil. The contraction of the surface on the other sides of the drop causes the oil to protrude at the weakest point much like a pseudopod. By using the proper kinds and strengths of material very close imitations of the movements of an Amoeba can be produced, but there are important differences presented by the behavior of a real Amoeba which it is difficult to interpret according to the surface tension theory. In a living Amoeba pseudopodia may be bent back and forth, thus showing that they possess some degree of rigidity; and when they are being withdrawn into the body, the outer layer often becomes wrinkled, which we should not expect it to do if its surface tension were the cause of the withdrawal. It is probable that the seat of contractility is not limited to the merest surface film of the ectoplasm; the whole ectoplasm apparently contracts much like muscular tissue. The protoplasm of an Amoeba when it passes from the sol to the gel state becomes more contractile, and it is probable that the changes in the viscosity of the outer protoplasm play an important part in the movements of the organism.

Amoeba, like numerous other primitive organisms, manages to perform its several functions without the aid of specialized organs. The exchange of substances involved in respiration is carried on through the entire surface of the body. Circulation is adequately provided for by the irregular movements of the fluid endoplasm. Excretion doubtless occurs through the exterior surface, but there is a small organ called the contractile vacuole also_concerned with this function. This vacuole as it becomes