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SIZE: 4.0 - 6.0 mm
TIME PERIOD: 26- 30 days post-ovulationn
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.
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 digestive epithelium layer begins to differentiate into the future locations of the liver, lung, stomach and pancreas.
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