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Stage 12 embryos have a greatest length of 3 to 5.4 mm and an estimated postfertilization age of 29 to 31 days. There are 21 to 29 body segments (somites) along the body axis. Most of the cephalic neural crests can be identified. The sinu-atrial openings and atrioventricular canal are present in the heart. The aortic arches are present and the dorsal aortae are fused. Four pharyngeal arches, grooves and pouches are present. Future hepatic sinusoids are occupied by hematopoetic tissue.


New structures include interventricular foramen, mesencephalon, M1, pharyngeal pouches, telencephalon medium, ultimopharyngeal body, vestibulocochlear neural crest (CN VIII).

Section 264

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