Summary
The dorsoventral axis of the human embryo is first definable at stage 3 (4 post-ovulatory days) on the appearance of a segmentation cavity in the morula. Attempts have been made to define the rostrocaudal axis at stages 5 c (11–12 days) and 6 (13 days) from the shape of the embryonic disc (narrower end caudal), but unequivocal demonstration awaits the initial appearance of the primitive streak during stage 6. During stages 6 to 8, the various median features appear. These are, from caudal to rostral: allantoic diverticulum, cloacal membrane, primitive streak, primitive node, notochordal process and chordal canal, and prechordal plate. At stage 8 (18 days), an early neural groove may also be visible. The bilateral symmetry of the embryo is interrupted in the heart during stage 10 (22 days), or perhaps already at stage 9 (20 days).
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O'Rahilly, R. The manifestation of the axes of the human embryo. Z. Anat. Entwickl. Gesch. 132, 50–57 (1970). https://doi.org/10.1007/BF00520952
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DOI: https://doi.org/10.1007/BF00520952