A first book in organic evolution

by D. Kerfoot Shute

Fig. 3.—Amœba proteus: n, nucleus; cv, contractile vesicle; ec, ectoplasm; en, endoplasm; p, pseudopodia.

In the periphery of the granular endoplasm, and adherent to the inner surface of the ectoplasm, is a clearly defined nucleus (n). When most distinctly seen, it presents the appearance of a clear vesicle surrounding a solid and more or less spherical nucleolus. A contractile vacuole (cv) is also uniformly present, located in the endoplasm. The creature has the power of putting out projections (p) from the surface called false feet (pseudopodia). Sometimes the protrusion consists of ectoplasm alone, but more commonly endoplasm extends into it, when a current of granules will be observed moving from the more central portions of the Amœba into its protrusion, whilst from some other protrusion that is being withdrawn a similar current may set towards the center of the body, and thus the animal moves, in a creeping manner, from place to place. While moving about in this way the little animal comes across other one-celled creatures, such as Desmids and Diatoms, seizing them and forcing them through its ectoplasm into the endoplasm, where the nutritious parts are digested and assimilated. After the animal has taken its prey through its ectoplasm, no break in the continuity of the ectoplasm remains, but the parts immediately come together in a perfect manner. After it has abstracted all the nutriment from its prey, the Amœba casts away from it the parts that are indigestible.

Fig. 4.—Rotalia Freyeri: a many-shelled Foraminifer, or a colony of many single-shelled Foraminifera, with pseudopodia extended.

Foraminifera. These are little protoplasmic unicellular animals that have the power of secreting for themselves more or less complex envelopes composed of limestone. They may be single, as in Lagena, or composed of a number of individuals with the shells cemented together as in Globigerina or Rotalia ([Fig. 4]). They have played a part of vast importance in the geological development of the world. Their myriads of shells remaining at the bottom of seas millions of years after the little protoplasmic bodies have perished, they have been consolidated into vast expanses of limestone rocks, and finally uplifted into such formations as the huge chalk cliffs of England.

Osteoblasts and Osteoclasts. These cells are naked pieces of protoplasm, the latter much the larger and having many nuclei. They are concerned in some of the most interesting phenomena of many growing animals. Just as the Foraminifera have the power of forming complex aggregations of limestone shells, so the Osteoblasts have the power to construct the bones of animals. And when a bone is broken as the result of accident, these little cells do the mending. While the Osteoblasts are bone-formers, the Osteoclasts are bone-destroyers. It is very curious that little specks of living jelly, like these Osteoclasts, should have the power of destroying hard tissue like bone, but such is the fact. These Osteoclasts can, by their wonderful chemical processes, liquefy and absorb, and by these means destroy, ivory pegs that are driven into living bone. They are the agents by which the roots of children’s milk teeth are destroyed, so that the crowns of the teeth are shed and the way paved for the appearance of the permanent teeth. The wonderful activity of these little Osteoblasts and Osteoclasts is well exemplified in the growth and shedding of the antlers of deer. While these antlers are growing in the spring, they are covered with a delicate skin, technically called “velvet.” This velvet is very sensitive and quite warm from the nutrient blood circulating through it. In it are hundreds and thousands of busy, living Osteoblasts that work together under some mysterious, directing or coördinating agency, to build up the splendid beams, tynes and snags that constitute the antlers, which in many deer of the Rocky Mountains reach such a size that a man may walk under the archway made by setting the shed antlers up on their points. No hive of bees is busier or more replete with active life than the antler of a stag as it grows beneath the warm, soft velvet, through the agency of the Osteoblasts.

The building of the antlers by these little agents continues through the spring and summer. In the autumn the Osteoblasts cease their activity and die; the delicate, sensitive velvet dries and peels off, leaving the dead, hard, bony substance exposed, and they now become weapons adapted for fighting. This is the season when the stags challenge one another to single combat, the hinds standing timidly by to be taken by the victor as his mates. When the loves and battles of the autumn are over and the mating is completed, the antlers no longer serve a useful purpose, and they are shed. The shedding is accomplished through the agency of the bone-destroyers, the little jelly-like cells called Osteoclasts.

Bacteria are exceedingly minute specks of naked protoplasm. They are unicellular plants. Some of them are harmless to mankind; some are very useful to him, and others are his deadly enemies. Many of them are concerned in the production of the infectious diseases. They do so by elaborating various chemical products that are virulent poisons, hence these products are called toxines; when taken up by the blood, they are carried to various parts of the body. In this manner they cause the particular symptoms that are characteristic of a special infectious disease. Why is it that some persons, on exposure to an infectious disease, contract the malady while others similarly exposed do not? In other words, what gives immunity to disease? The explanation is probably as follows: Just as the invading bacteria have the power of secreting toxines, so the cells of the body, normally, have the power of elaborating chemical products that are antidotes to the toxines, and are appropriately called antitoxines. Infectious diseases and immunity from them, are the result of a contest between the invading bacteria and the protecting cells of the body. If the bacteria secrete toxines in greater quantities than can be neutralized by the cells of the body, we have disease; if the reverse occur, we have immunity.

The white blood-corpuscles (cells) also take part in this warfare. They have the power of traveling, in virtue of their amœboid movements, from the blood to the part invaded by the bacteria. Here a contest takes place between them, the corpuscle takes the bacteria into its interior, and either kills them or is itself killed. The result of this contest helps to produce either immunity or infection.