References

O'Rahilly, R. and F. Müller, Developmental Stages of Human Embryos, Carnegie Institution of Washington, Publication 637, 1987.

Streeter, G.L. (1951) Developmental Horizons in Human Embryos; Description of Age Groups XIX, XX, XXI,XXII, and XXIII being the Fifth Isssue of a Survey of the Carnegie Collection, Carnegie Institution of Washington, Publication 592, Contributions to Embryology, 34, 165 - 196.