The embryo described above had 15 pairs of somites and, therefore, belongs to Streeter’s developmental horizon XI (Carnegie stage 11), comprising the embryos with 13–20 pairs of somites. Streeter (1942) first made a review of horizon XI based on 16 embryos. Recently Müller and O’Rahilly (1986) surveyed 20 embryos belonging to stage 11 with special reference to the development of the central nervous system, 15 of which were once dealt with in Streeter’s review. O’Rahilly and Müller (1987) further published a comprehensive study on human embryos from stage 2 to stage 23, in which 23 embryos belonging to stage 11 were investigated.
KeywordsNeural Plate Primitive Streak Sinus Venosus Coelomic Cavity Septum Transversum
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