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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.