3B Scientific Embryonic Development Model, 12 stages - Handbuch (Seite 4)

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10. On the 20th day the extro-embryonic mesenchyme is partially decomposed. It is preserved, in the
neighbourhood of the embryo, os covering mesenchyme, and at the inner edge of the trophoblast,
os border mesenchyme. Between them lies the chorionic cavity. A tissue bridge, the body stolk, os
primordium of the umbilicol cord extends from the trophoblast to the embryonic primordium. The
embryonic allantois, 0 protrusion of the vitelline soc, grows into the peduncle.
11. The body stalk displaces with the increase of the omniotic cavity towards the middle of the embryo,
and becomes an abdominal peduncle. Blood vessels develop at the vitelline sac and in the abdominal
peduncle. In this stage the embryo shows 4 branchial furrows which degenerate again, later on the
primordia 0f eyes, arms, legs and at the back the primitive segments of the mesoderm, from which
originate the vertebral column, muscles, blood-vessels and kidneys.
12. In the 2nd month the amniotic cavity - it contains liquid to protect the embryo - is increased and sup-
plants the chorionic cavity. The umbilical cord is formed by those parts of the vitelline sac that, Iying
outside of the embryo, are in retrogressive metamorphosis, and by the abdominal peduncle with the
blood-vessels. The chorionic villi grow at the trophoblast, particularly at the insertion of the umbilical
cord (shaggy chorion, the primordium of placenta). In the other sections these villi perish (smooth
chorion). In the embryo the head increases by rapid growth of the primordium of the brain. The pri-
mordia of extremities largely extend forward.