CREATION BY EVOLUTION
resemblances are very striking. For example, in groups as distinct as flat-worms, annelids, and mollusks corresponding cleavage cells give rise to similar organs, and the larvae of these forms are very much alike, though the adult forms are very different.
In all vertebrates, from Amphioxus to man, there are many fundamental resemblances which distinguish this one phylum from all others. The eggs of vertebrates vary greatly in size, ranging from the microscopic eggs of mammals to the enormous ones of birds, and they differ also in the conditions under which they develop. Some are set free as naked cells to begin their lives independent of their parents; others are enclosed in protective membranes and shells and are further guarded and incubated by the mother or father; and still others undergo their development within the body of the mother. Associated with these varying conditions of development are many differences in the size of eggs, the rate of development, the methods of nutrition and other features. But the striking fact remains that in all vertebrates, from the lowest to the highest, one finds fundamental agreement in the position and structure of the principal organs of the body, and these organs arise from corresponding parts of the egg or embryo in essentially the same manner in all. Thus the brain and the nervous system come from a plate of superficial cells on the dorsal surface of the gastrula — a plate that rolls up at the sides to form a groove and then a tube. The front end of this tube enlarges to form the brain; the hinder part forms the spinal cord. The backbone, which gives the name to the phylum (vertebrate or chordate) appears as a row of cells, the notochord, above the alimentary canal and below the nerve plate. This row of cells is the basis of the backbone; it becomes surrounded by cartilage, is then changed to bone and is segmented into the vertebrae of the
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