. Principles of modern biology. Biology. Reproduction in Multicellular Animals - 38V AMNIONIC CAVITY EXTRA- EMBRYONIC COELOM. ALLANTOIS EXTRA- EMBRYONIC COELOM PLACENTA UMBILICAL ARTERY UMBILICAL VEIN •AMNIONIC CAVITY Fig. 21-11. Development of the embryonic membranes and the placenta in man and higher mammals generally. Compare with membranes of the bird's egg (Fig. 21-12), noting differ- ences in the amnion, allantois, and yolk sac. oviparous (egg-laying) habits of their aquatic ancestors. Eggs laid on land, however, are al- ways covered by a shell and other protective membranes, for otherwi
. Principles of modern biology. Biology. Reproduction in Multicellular Animals - 38V AMNIONIC CAVITY EXTRA- EMBRYONIC COELOM. ALLANTOIS EXTRA- EMBRYONIC COELOM PLACENTA UMBILICAL ARTERY UMBILICAL VEIN •AMNIONIC CAVITY Fig. 21-11. Development of the embryonic membranes and the placenta in man and higher mammals generally. Compare with membranes of the bird's egg (Fig. 21-12), noting differ- ences in the amnion, allantois, and yolk sac. oviparous (egg-laying) habits of their aquatic ancestors. Eggs laid on land, however, are al- ways covered by a shell and other protective membranes, for otherwise the developing embryos could not survive the drying effects of the atmosphere. Moreover, the embryos of land animals reach a relatively high state of development before they are able to face the hazards of the land environment. Accord- ingly, the egg cells of birds and reptiles, be- sides containing relatively large amounts of yolk, are surrounded by an "; This albuminous fluid provides the developing embryo with an extra reserve of water and additional protein reserves. Both the "egg- white" and the calcareous shell are secreted by the glandular walls of the oviducts, after ovulation has occurred, while the egg is pass- ing through the oviduct. The Embryonic Membranes. In reptiles, birds, and mammals, embryonic development proceeds in the absence of the age-old aquatic environment, and such development involved the evolution of a highly specialized system of embryonic membranes. These arose in the form of the amnion and the allantois, a pair of living membranes that grow out from the embryo, enveloping it completely. Jointly these membranes, together with the yolk sac (Fig. 21-12), protect the embryo and sustain it by absorbing oxygen and foods and by eliminating metabolic wastes during the ex- tended period of development. The Amnion. The amnion, as it first ap- pears, is an outfolding of the body wall of the embryo, which carries with it an
Size: 1469px × 1700px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodiversity, booksubjectbiology
