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Fig. 5.—Gastrulation and formation of the entoderm in the three types of chordate eggs. All shown in median sagittal section. A, Amphioxus (Branchiostoma); 1, beginning of the invagination; 2, invagination completed; 3, completed gastrula having a wall of two layers, ectoderm and entoderm, and an internal cavity, the gastrocoel. B, amphibian; 1, beginning of the invagination; 2, progress of the invagination accompanied by downward growth of the ectoderm; 3, completed gastrula, with very thick entoderm ventrally. C, reptile or bird; 2, posterior margin of the blastoderm beginning to turn under; 2, con- tinuation of the invagination; a slight split between entoderm and yolk constitutes the gastrocoel. In A3, B3, and C2 the anterior end of the embryo is to the left, posterior end to the right, dorsal surface above, ventral below. (A from Parker and Haswell’s Textbook of Zoalogy, after Hatschek, courtesy of the Macmillan Company; B from Kellicott’s General Embryology, courtesy of Henry Holt and Com-