The New Student's Reference Work/Development of Animal Life
Development of Animal Life. The building of an animal's body is the most wonderful thing in all nature. An insect, a fish or a bird begins its development as an egg, and, as the construction of the body goes on, each tissue and each organ are formed anew out of the material contained within the egg. After three weeks' incubation of the hen's egg, for example, the young chick steps into the world with heart, brain, eyes and other organs all formed—a remarkable transformation. Frogs' eggs, laid in the water, undergo similar changes without any care from the parent; tadpoles hatch from them, and in due course of time these tadpoles grow into frogs, with a different kind of body. The hen's egg is large, because there is a large quantity of food-yolk stored up for the use of the growing chick; the frog's egg is smaller, because it contains less yolk; and some eggs—for example, those of starfishes—are smaller than pin-heads. The true starting-point of the chick is a microscopic cell within the egg; and, when we look to other animals, we find that all of them, no matter how complex, start in the condition of a single microscopic cell and that, between that simple state and the fully formed animal, which is complex, there are many steps. Therefore, the adult stage of any animal represents the last step in a long series of modifications. If we could only follow the changes, step by step, we should have a means of understanding all about the construction of animals and their past history. Tracing the stages by which cells emerge into tissues, tissues into organs and how the organs by combination build the body is called embryology or development. It is an important fact to keep before us that the rudiment of all life is a cell. (See Cell-Doctrine.) If we look upon cells as the bricks of organic architecture, the starting-point of a many-celled animal is a single brick; but, inasmuch as each egg needs to be fertilized before developing—just as a plant-ovule must be fertilized by pollen before it becomes a seed—the single brick is a compound one, made of material derived from each parent. The development of all animals is remarkably alike; from the single cell there come, by division, many cells; these continue to feed on the yolk, to grow and divide; and thereby a large number of cells arises. These cells arrange themselves into definite layers, from which all parts of the body are formed.
The earlier view of development was that the animal existed already formed within the egg, but was exceedingly minute, and that development consisted in the expansion or growth of this animal in miniature. But William Harvey (1598-1657) and Caspar F. Wolff (1733-94) showed the falsity of this view. The latter especially in 1759 showed (in Theoria Generationis) the true nature of development to consist in a real becoming or gradual formation, step by step, of the organs and the animal's body. This, of course, was opposed to the view that the embryo was preformed. The acceptance of his work was long delayed on account of the opposition of Haller the great physiologist and of others, but in 1812 it began to receive notice after the long period of neglect, and in the course of a few years the truth that Wolff contended for was triumphantly established. This was the first epoch in modern embryology.
The second epoch was created by K. E. von Baer in showing that all the tissues and organs come from cell-layers, or germ-layers. Baer (1792-1876) is regarded as the father of modern embryology. In 1828 he published his great work on the development of animals, in which he showed that the numerous cells, produced by division of the original cell, become arranged into three layers—an outer, a middle and an inner layer—and that this is true for all animals above the very lowest. These layers are called the germ-layers, and each one gives rise to a particular set of tissues and organs. For example, from the outer germ-layer there comes, in all animals, the nervous system, including the brain, spinal cord, nerves and sense-organs; the outer covering of the body with parts like scales, feathers, hair. The middle layer splits into two sheets and is very complex. It gives rise, in all animals, to muscles, heart, blood-system, connective tissue, including cartilage and bone, etc. The inner layer forms the lining of the alimentary tube, etc. It is a remarkable fact that the organs of the many different kinds of animals are essentially alike in origin. This fact unites them all on a broad plane and is of great meaning in understanding their history. It is called the germ-layer theory. Modern embryology has largely become a study of the origin and history of the germ-layers.
Another epoch of advance is marked by the work of Kowalevsky, who in 1866 broke down by embryological study the rigid line that was supposed to separate invertebrated and vertebrated animals, and thereby brought them closer together. In 1881 Balfour, one of the greatest of modern embryologists, brought together all that was known about the development of animal life, from sponges to the highest animals, and published it in his Comparative Embryology. From that time the advance of knowledge about this subject has been very great, and embryology is looked upon as one of the most important of the biological sciences. It enables us to get at the past history of animals, and throws much fight upon their relationships one with another. All animals have had a history. Development shows what that history has been. There is good reason to believe that the higher animals have been derived gradually from the simpler ones. It follows that the higher ones had simpler ancestors, who might have lived very far in the past. In the course of their development animals repeat to a certain extent the story of their past, and, therefore, certain traces of ancestral organs make their appearance. Let us take, for example, the chick developing in the hen's egg. This is a bird; nevertheless, gill-clefts that belong to fishes and smack of a water-life, make their appearance in the developing bird and then fade away. The heart is also, at the same time, two-chambered as in the fish, and the blood vessels are arranged in the gill-arches as in the fish. These rudimentary organs are not of use to the young bird for breathing, but their presence means something; they are not there by chance. The best explanation seems to be that they are inherited from the remote ancestors of the birds, which were water-breathers. Many other rudimentary organs arise and disappear, and are explained in a similar manner. It is an astounding fact that the gill-clefts arise in all of the highest animals, without exception, and their presence is taken to indicate that the remote ancestors of all were water-breathers and had use for gills. The rudimentary organs referred to of course disappear long before the animal is hatched or born into the world. These structures give clues to former conditions, and the traces we find are like records left by the hand of time upon the embryo. The reading of this embryological record is like reading the hieroglyphics and inscriptions made by ancient people upon monuments, temples and columns, but the inscriptions on the embryo go farther back into the past. In like manner, the stages of the frog show its history. The tadpole is at the level of a fish, but it undergoes further changes and transforms into an air-breathing animal. Now, since the most complex organs of animals start in a state of simplicity, it follows that, if we take them in their simplest stages and see all the modifications as they are added, we shall be better able to understand them. This is of wide application, and shows why embryology is so important in zoology and also in botany. Observations on development throw more light on animal-life than comes from any other single source.