Chorioallantoic membranes (embryonic membranes) form of cells layers that are in the direct site of attachment of the embryo to the uterine wall (dia

Formation of chorioallantoic membranes [embryonic]

Chorioallantoic membranes (embryonic membranes) form of cells layers that are in the direct site of attachment of the embryo to the uterine wall (diagram 127). The cells of the outer layer of the embryo proliferate, in them sprout blood vessels and form secondary villi, which connect with the mother's blood vessels, subsequently forming an afterbirth, or placenta. Starting from the second week forms a thin shell of amniotic sac that surrounds the embryo (diagrams 128, 129). Its inner cells produce fluid — amniotic fluid (diagram 129), in which “floats” the embryo. Fluid protects fetus from mechanical damage. By the time of the birth of the child accumulates 1-1.5 liters of amniotic fluid. At 10-14 days after fertilization, of the cells of embryonic shield form three layers (three germ layers), which later form all organs and tissues (diagram 127). At 16-18 day blood vessels form in embryo, heart starts to work, appear rudiments of ears, eyes and the brain. At the end of six weeks it’s already showing neural tube, notochord, intestine, and so on. At the same time in the embryo, which is no more than a grain of rice (diagram 126), appear the rudiments of arms and legs.

Diagram 126. Stages of the embryo and fetus development
Diagram 127. A simplified diagram of interposition of the human embryo (2-3 weeks) and chorioallantoic membranes: 1 — external chorioallantoic membranes (chorion) with outgrowths; 2 — internal chorioallantoic membranes (amnion), from which subsequently forms the amniotic sac; 3 — the liquid that fills the amniotic sac; 4 — ingrown in endometrium chorionic villi from which subsequently forms the placenta; 5 — cell of embryonic shield, consisting of the three germ layers; 6 — the umbilical cord; 7 — the uterine cavity; 8 — lining of the uterus (endometrium)
Diagram 128. Human fetus at 9-10 weeks
Diagram 129. The placenta and the umbilical cord: 1 — uterine wall; 2 — the fetus; 3 — the umbilical cord; 4 — placenta; 5 — the blood vessels; 6 — umbilical artery; 7 — umbilical vein; 8 — the umbilical cord; 9 — amniotic fluid
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Questions:
  • Explain the biological significance of chorioallantoic membranes and germ layers.