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2. Spemann, H., 1938, Embryonic Development and Induction. Yale University Press, New Haven, CT.
3. Spemann, H., and Mangold, H., 1924, Ueber Induktion von Embryonalanlagen durch Implantation artfremder Organisatoren, Arch. mikr. Anat. Entwicklungsmech. 100:599–638. English transi. in Foundations of Experimental Embryology, 2nd ed., 1974 ( B. Willier and J. Oppenheimer, eds.), Hafner Press, New York, pp. 146–184.
4. Spemann, H., 1903, Entwicklungsphysiologische Studien am Tritonei III, Wilhelm Roux’ Arch. Entwicklungsmech. 16: 551–631.
5. Holtfreter, H., and Hamburger, V., 1955, Embryogeneses: Progressive differentiation. Amphibians, in: Analysis of Development (B. H. Willier, P. A. Weiss, and V. Hamburger, eds.), Saunders, Philadelphia, pp. 230–296. Quotation from p. 275. Emphasis by original authors.