Henry Gray (18251861). Anatomy of the Human Body. 1918.
FIG. 14 Surface view of embryo of Hylobates concolor. (After Selenka.) The amnion has been opened to expose the embryonic disk. (See enlarged image)
FIG. 15 Series of transverse sections through the embryonic disk of Tarsius. (After Hubrecht.) Section I passes through the disk, in front of Hensens knot and shows only the ectoderm and entoderm. Sections II, III, and IV pass through Hensens knot, which is seen in V tapering away into the primitive streak. In III, IV, and V the mesoderm is seen springing from the keel-like thickening of the ectoderm, which in III and IV is observed to be continuous into the entoderm. (See enlarged image)
Ectoderm.The ectoderm consists of columnar cells, which are, however, somewhat flattened or cubical toward the margin of the embryonic disk. It forms the whole of the nervous system, the epidermis of the skin, the lining cells of the sebaceous,