REPRODUCTION 131 



38. Protection of the Embryo 



The egg and the developing embryo are in general very susceptible 

 to unfavorable environmental conditions and a variety of adaptations 

 have evolved in invertebrates and vertebrates to tide the embryo over 

 this critical period. 



The eggs of many parasitic worms are covered with shells which 

 enable them to survive exposure to heat, cold, desiccation and digestive 

 juices. The skate egg is covered by a tough, leathery case that protects 

 the developing embryo within. The eggs of most fish and amphibia are 

 surrounded by a jelly coat which is of some value in protecting against 

 mechanical shock. The eggs of reptiles and birds are protected by tough 

 leathery or calcareous shells. The developing chick embryo "breathes," 

 takes in oxygen and gives off carbon dioxide, through its shell. 



The eggs of fish and amphibia are fairly large and contain yolk 

 which supplies the nutrients for the developing embryo. These eggs are 

 laid and typically develop in water, whence the oxygen, salts and water 

 required for development are obtained. The embryos develop a pouch- 

 like outgrowth of the digestive tract, the yolk sac, which grows around 

 the yolk, elaborates enzymes to digest it, and transports the products in 

 its blood vessels to the rest of the embryo. 



The eggs of reptiles and birds develop on land rather than in water, 

 and further adaptations were required to permit development in the 

 absence of the large body of water. These forms have three additional 

 membranes, the amnion, chorion and allantois, which are sheets of liv- 

 ing tissue that grow out of the embryo itself. The amnion and chorion 

 develop as folds of the body wall and surround the embryo; the allantois 

 grows out of the digestive tract and functions along with the yolk sac 

 in nutrition, excretion and respiration. Each of these membranes is com- 

 posed of two germ layers in close apposition (Fig. 6.10). 



The formation of the amnion is a complex process and its details 

 differ in different animals. A bilateral, double-walled outfolding of the 

 body wall of the embryo grows upward and medially to surround the 

 embryo and fuse above it, enclosing a space, the amniotic cavity, be- 

 tween itself and the embryo. This is filled with a clear, watery fluid 

 secreted in part by the embryo and in part by the amnion. The amnion 

 develops from the inner part of the original fold; the outer part be- 

 comes the second fetal membrane, the chorion, which lies outside of 

 and surrounds the amnion. The chorionic cavity, also known as the 

 extraembryonic coelom (for the space is continuous with the coelomic 

 cavity within the embryo), is the space between the amnion and chorion. 

 The embryos of reptiles, birds and mammals develop in the liquid-filled 

 amniotic cavity, their own private pond within the shell or uterus. This 

 arrangement permits the embryo to move around to some extent but 

 protects it from bumps and shocks. The chorion of reptiles and birds 

 comes to lie next to the shell and that of mammals is in contact with 

 the maternal tissues of the uterus. The allantois is an outgrowth of the 

 digestive tract which grows between the amnion and chorion and largely 

 fills the chorionic cavity. The allantois of the bird and reptile typically 



