Page:Anatomy of the Human Body (1918).djvu/48

48 Fig. 14.—Surface view of embryo of Hylobates concolor. (After Selenka.) The amnion has been opened to expose the embryonic disk.

Fig. 15.—Series of transverse sections through the embryonic disk of Tarsius. (After Hubrecht ) Section I passes through the disk, in front of Hensen's knot and shows only the ectoderm and entoderm. Sections II, III, and IV pass through Hensen's 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.