DEVELOPMENT OF THE ORGANISM
ancestor and diverged from one another owing to their adoption of different habits; and, strangest of all, that backboned animals, the class to which we ourselves belong, diverged from the same root as the starfish, sea-urchins, and sea cucumbers, which constitute the class Echinodermata.
Comparative anatomy and systematic zoölogy take us only a little way, for we have no reason to assume that the ancestral forms of animals have persisted unchanged to the present day. The evidence from fossils is best, but fossils preserve only the hard parts, and the earliest fossils thus far found are already far advanced in evolution. But every animal begins its development in the egg, which is a single cell, comparable in structure to the lowest forms of life known to us, and as it grows to the adult form it sketches in broad outlines the whole story of its evolution.
REFERENCES
There are unfortunately no short, comprehensive books on embryology, and the reader who wishes to pursue the subject further must have recourse to large treatises, in which it is dealt with in detail. For the development of the invertebrates we recommend Textbook of Embryology (vol. 1), Invertebrates, by E. W. MacBride ; Macmillan & Co. For the development of the vertebrata we recommend The Embryology of the Vertebrata (this book does not include the frog) and The Frog, both by A. Milnes Marshall; for The Embryology of the Chick, Frank R. Lillie. Text books of embryology by Clement Heisler and by Bailey and Miller deal with the embryology of man.
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