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 "egg-white." 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 exten- 

 sion of the coelomic cavity (Fig. 21-11 A). 

 This double membranous fold continues to 



