EMBRYOLOGY AND EVOLUTION
vertebral column. In all vertebrates the right and the left side of the pharynx — the cavity behind the mouth and nose — are pierced by a series of gill slits, and between these slits run arterial arches, which in fishes and water-breathing amphibians bring blood to the gills to be aerated; but in all air-breathing vertebrates true gills are lacking, though the arterial arches are still present in the embryo, and they undergo characteristic modifications in different classes of vertebrates. This very brief and general statement of the course of embryonic development applies to all vertebrates, man included.
In all animals development begins with the fertilized egg, which contains none of the structures of the developed animal, though every egg does contain specific kinds of protoplasm, which differ not only in different phyla and species but even in different individuals of the same species. These different kinds of protoplasm in the germ cells constitute the material basis of inherited differences in mature organisms, and the more unlike animals are in their adult state the more unlike is this germinal protoplasm of the egg cells from which they develop. In all organisms the egg contains hereditary units by whose combinations and transformations the characters of the cleavage, blastula, gastrula, embryo, and adult are formed. Development consists in such combinations and transformations, and just as the chemical combination of hydrogen and oxygen to form water gives rise to a substance with new qualities which were not present in the elements that entered into the combination, so new structures and functions develop out of germ cells — structures and functions that were not present as such in the cells. This is "creative synthesis," or "creative evolution," and it is a phenomenon that is found in all nature.
The development of functions goes hand in hand with the
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