EMBRYOLOGY AND EVOLUTION
The Course of Development
Let us consider in brief outline some of the main facts of individual development. In practically all animals and plants development begins with the fertilization of an egg. William Harvey (1651), the discoverer of the circulation of the blood, expressed this fact in his famous dictum, “Omne vivum ex ovo.” The egg is a cell with the structures and functions that are characteristic of cells in general; that is, it contains protoplasm, "the physical basis of life," and this protoplasm is differentiated into a nucleus and a cellbody, and each of these contains other smaller units, many of which differ one from another. But these units are not adult parts in miniature, as the “preformationists” supposed; they are merely the elements out of which adult parts are built; they are like the letters of the alphabet out of which are built words and books and literatures.
In all animals the process of fertilization is practically the same, consisting essentially in the union of the nuclei of egg and spermatozoon. In almost all multicellular animals the egg at or near the time of fertilization gives off two minute cells, the polar bodies, which are rudimentary eggs and take no part in development. The fertilized egg undergoes repeated divisions or cleavages, forming a mass of cells, usually a hollow sphere, which is called a blastula, and this in turn becomes a gastrula by the formation in it of a gastric cavity. So far all animals, from sponges to men, travel the same road, although in every group there are minor peculiarities; they travel the same road, but they do not all follow in exactly the same tracks. The germ cells, the cleavage, the blastula, and the gastrula show characteristic differences in different phyla or classes, but their resemblances are more significant than their differences, and within the same phylum these
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