Friedman, W. F., and Silverman, N. Congenital heart disease in infancy and childhood. In Heart Disease, sixth edition. E. Braunwald, D. P. Zipes, and P. Libby, eds. W. B. Saunders, Philadelphia, 2001.
Kalousek, D. K., and Hendson, G. Embryofetopathology. In Encyclopedia of Human Biology, second edition, Academic Press, San Diego, 1997, p. 605.
Shepard, T. H. Teratology. In Encyclopedia of Human Biology, second edition, Academic Press, San Diego, p. 253, 1997.

TABLE 1 Developmental Stages in Human Embryos a

Age (days) Stage No. of somites Length (mm) Main characteristics
20–21 9 1–3 1.5–3.0 Deep neural groove and first somites present. Head fold evident.
22–23 10 4–12 2.0–3.5 Embryo straight or slightly curved. Neural tube forming or formed opposite somites, but widely open at rostral and caudal neuropores. First and second pairs of branchial arches visible.
24–25 11 13–20 2.5–4.5 Embryo curved due to head and tail folds. Rostral neuropore closing. Otic placodes present. Optic vesicles formed.
26–27 12 21–29 3.0–5.0 Upper limb buds appear. Caudal neuropore closing or closed. Three pairs of branchial arches visible. Heart prominence distinct. Otic pits present.
28–30 13 30–35 4.0–6.0 Embryo has C-shaped curve. Upper limb buds are flipper-like. Four pairs of branchial arches visible. Lower limb buds appear. Otic vesicles present. Lens placodes distinct. Attenuated tail present.

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