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2. Noyes, F.B. ; Schour, I.; and Noyes, H.J.: Dental Histology and Embryology, 5th ed, Philadelphia: Lea and Febiger, 1942 , p 92.
3. Orban, B.: Oral Histology and Embryology, H. Sicher (ed), 5th ed, St. Louis : C. V. Mosby Co., 1962, p 55.
4. Provenza, D.V. : Oral Histology, Inheritance and Development , Philadelphia: J. B. Lippincott Co., 1964, p 200.