Fertilization to the End of the Eighth Week
The major contributions to our understanding of basic morphogenesis in early human development were made during the last century. One of the best classic descriptions of human organogenesis can be found in the Manual of Human Embryology, edited by Keibel and Mall in 1910 and 1912. The first formal classification of human embryos was published by Mall in 1914. At the present time, the development of human embryos, from fertilization until the embryo has attained a crown-rump (CR) length of 30 mm, is divided into 23 stages (O’Rahilly and Muller, 1987; Streeter, 1951). After implantation, the embryo is characterized by the number of somites, whereas in later stages, the size of the embryo or its specific external features become important for staging. The main external features of each stage are summarized in Appendix II (Table II-1). This classification allows an easy, accurate evaluation of normal human embryos.
KeywordsHuman Embryo Chorionic Villus Branchial Arch Primitive Streak Placenta Praevia
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