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1. What are extraembryonic membranes? Extraembryonic membranes are membranous structures that appear in parallel with the embryo and play important roles in the embryonic development. They form from the embryo but do not become part of the individual organism after its birth. 2. What are the extraembryonic membranes present in vertebrates? The extraembryonic membranes that may be present in vertebrates are the yolk sac, the amnion, the chorion, the allantois and the placenta. 3. Are the extraembryonic membranes the same in all vertebrates? The presence of each extraembryonic membrane varies according to the vertebrate class. In fishes and amphibians only the yolk sac is present. In reptiles and birds besides the yolk sac there are also the amnion, the chorion and the allantois. In placental mammals besides all these membranes the placenta is present too. 4. How is the yolk sac formed? What is the function of the yolk sac? The yolk sac is formed from the covering of the vitellus by some cells originated from the primitive gut. The yolk sac stores vitellus, the main nourishment source of non-placental embryos. 5. Which is the extraembryonic membrane whose function is to store nitrogen wastes of the embryo? Is this function present in placental mammalian embryos? The allantois is the extraembryonic membrane whose function is to store the excreted matter of the embryo. In placental mammals the allantois is present but it does not exert that function since the embryonic wastes are collected by the mother’s body through the placenta. 6. Why can the allantois be considered an adaptation to terrestrial life? The allantois is an adaptation to dry land because in embryos of oviparous terrestrial beings, like reptiles and birds, the metabolic residuals cannot be immediately excreted to the aquatic surrounds (as fishes and amphibian larvae do). It was necessary then for the appearance of a structure capable of storing the embryonic excretes until hatching. 7. What is the difference between amnion and chorion? Amnion is the membrane that covers the embryo. Chorion is the membrane that covers the amnion, the yolk sac and the allantois. The space delimited by the chorion and the amnion is called amniotic cavity and it is filled with aminiotic fluid. The amniotic cavity has the functions of preventing desiccation of the embryo and of protecting it against mechanical shocks. 8. Why can the amnion also be considered an adaptation to terrestrial life? The amnion is also an adaptation to dry land since one of its functions is to prevent desiccation of the embryo. 9. What is the chorioallantois membrane present in the embryonic development of reptiles and birds? How does this membrane participate in the energetic metabolism of the embryo? The chorioallantois membrane is formed by juxtaposition of some regions of the chorion and the allantois. Since it is porous, the chorioallantois membrane allows the passage of gases between the embryo and the exterior thus making aerobic cellular respiration possible. 10. In which type of animals does the placenta exist? What is its main function? True placenta is present in placental mammals. The placenta is formed from the chorion of the embryo and from the mother’s endometrium. Its main function is to allow the exchange of substances between the fetus and the mother’s body. 11. What are the main substances transferred from the mother to the fetus through the placenta? And from the fetus to the mother? From the mother to the fetus the main transferred substances through the placenta are water, oxygen, nutrients and antibodies. From the fetus to the mother, metabolic wastes, including urea (nitrogen waste), and carbon dioxide. 12. Is there a exchange of cells between the mother and the fetus through the placenta? Under normal conditions, there is no passage of cells across the placenta during gestation. The placenta has a smooth mucosa separating the richly vascularized region in contact with the mother’s endometrium from the umbilical cord in contact with the fetal blood. This barrier is known as placental barrier. Although permeable to some substances (selective permeability) the placental barrier forbids the passage of cells. 13. What are the endocrine functions of the placenta? The placenta has endocrine function since it secretes the hormones progesterone and estrogen that maintain the endometrium (internal covering of the uterus) and prevent menses during pregnancy. The placenta also secretes other important hormones for pregnancy regulation. 14. What is the function of the umbilical cord? The umbilical cord is a set of blood vessels that connects the fetus with the placenta. In the fetus one extremity of the cord inserts into the center of the abdominal wall (the later scar of this insertion is the umbilicus or navel). The function of the umbilical cord is to allow the transport of substances, nutrients, gases and residuals, between the fetus and the mother’s body. |