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A series of transverse sections through an embryo of the dog. (After Bonnet.) Section I is the most anterior. In V the neural plate is spread out nearly flat. The series shows the uprising of the neural folds to form the neural canal. a. Aortæ. c. Intermediate cell mass. ect. Ectoderm. ent. Entoderm. h, h. Rudiments of endothelial heart tubes. In III, IV, and V the scattered cells represented between the entoderm and splanchnic layer of mesoderm are the vasoformative cells which give origin in front, according to Bonnet, to the heart tubes, h; l.p. Lateral plate still undivided in I, II, and III; in IV and V split into somatic (sm) and splanchnic (sp) layers of mesoderm. mes. Mesoderm. p. Pericardium. so. Primitive segment.