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SIZE: 3.0 - 5.0 mm
TIME PERIOD: 25- 27 days post-ovulation
The embryo curves into a C shape. The arches that form the face and neck are now becoming evident under the enlarging forebrain. By the time the neural tube is closed, both the eye and ear will have begun to form. At this stage, the brain and spinal cord together are the largest and most compact tissue of the embryo.
Epipharyngeal placodes/discs are thickened areas of surface ectoderm and part of the ectodermal ring (O’Rahilly & Müller, 1985). They contribute cells to the cranial ganglia. The placodes appear in stage 12 and are particularly active in pharyngeal arch 1.
Placode activity of pharyngeal arch 1 differs from that in arches 2–4 as the ectodermal contribution appears earlier and is not a clearly localized thickened area. It may indicate differences in Hox gene expression found between arch 1 and the following arches (Vieille-Grosjean et al. 1997).
Arches 2–4 begin to appear in stages 12 through 13 (Van Campenhout, 1948). Increased activity of alkaline phosphatase has been detected in the placode of arch 3 at stage 12 (Mori, 1965).
A blood system continues to develop. Blood cells follow the surface of yolk sac where they originate, move along the central nervous system, and move in the chorionic villi, the maternal blood system.
Valves and septa may appear in the heart in Stage 12.
The beginning cells of the liver form before the rest of the digestive system.
Specialization of cells to become the intestinal tract, lungs, liver and kidneys begins.
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