Stage 11
Thirteen to Twenty Somite Pairs, Rostral Neuropore Closes, Optic Vesicle Appears, Two Pharyngeal Arches Appear
2.5 - 3.0 mm
23 - 25 days post-ovulation
http://www.visembryo.com/baby/stage11.html
Thirteen to twenty pairs of somites are present in Stage 11 and the embryo is shaped in a modified S curve. The embryo has a bulb-like tail and a connecting stalk to the developing placenta.
A primitive S-shaped tubal heart is beating and peristalsis, the rhythmic flow propelling fluids throughout the body, begins. However, this is not true circulation because blood vesel development is still incomplete.
At this stage, the neural tube determines the form of the embryo. Although the primary blood vessels along the central nervous system are connecting in Stage 11, the central nervous system appears to be the most developed system. If twenty somites are present in the embryo, the forebrain is completely closed.
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