Henry Gray (18251861). Anatomy of the Human Body. 1918. 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.