Transverse Section of an Embryo Chick
Transverse section of an embryo chick (third day). Labels: mr, rudimentary spinal cord; the primitive central canal has become constricted in the middle; ch, notochords; layer of mesoblast lining groove, which is not yet closed in to form the intestines; a o, one of the primitive aorta, u n, Wolffian body; ung; Wolffian duct; vc, vena cardinalis; h, epiblast; hp, somatopleure and its reflection to form af, amniotic fold; p, pleuroperitoneal cavity.
Baker, W. Morrant & Harris, Vincent Dormer Kirkes' Hand-book of Physiology, 13th ed. (Philadelphia: P. Blakiston's Son & Co., 1892) 844