1. O’Rahilly R. The timing and sequence of events in the development of the human digestive system and associated structures during the embryonic period proper. Anat Embryol (Berl). 1978;153:123–36.
2. Lewis F. The early development of the entodermal tract and the formation of its subdivisions. In: Keibel F, Mall FP, editors. Human embryology. Philadelphia: J.B. Lippincott; 1912. p. 295–334.
3. O’Rahilly R, Muller F. Developmental stages in human embryos. Washington, DC: The Carnegie Institution of Washington; 1987.
4. Sembra R, Tanaka O, Tanimura T. The digestive tract. In: Nishimura H, editor. Atlas of prenatal histology. Tokyo: Igaku-Shoin; 1983. p. 193–225.
5. Moxey PC, Trier JS. Endocrine cells in the human fetal small intestine. Cell Tissue Res. 1977;183:33–50.