Chap. 35 



REPTILES FIRST LAND VERTEBRATES 



715 



Fig. 35.2. Upper, embryo of the painted turtle, Chrysemys picta, enlarged about 

 3 times. Lower left, embryo of the snapping turtle, Chelydra serpentina, sur- 

 rounded by the amnion. Lower right, snapping turtles at hatching, about natural 

 size. Reptiles were the first land animals. Before them all animals had been 

 bound to the water. Their young could not and cannot now develop without it 

 but now they have it in a sac. The shelled egg and the amnion, the sac of fluid 

 that contains the embryo, were the keys to land life for the reptiles. Shelled eggs 

 are all important to the birds, and the amniotic sac of fluid has continued im- 

 portant in birds and mammals. Every human being spends his early months in a 

 pond. (After Agassiz: "Embryology of the Turtle," in Contributions to Natural 

 History of U.S.A., vol. II, pt. III. Boston, Little, Brown, and Co., 1857.) 



had been achieved by any animals before them. Many of the ancient dinosaurs 

 could run on their hind legs, and dig up roots, pick fruit and fight with their 

 forefeet (Fig. 35.15). Most snakes can travel rapidly and although they have 

 no appendages they can climb and swim. 



