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THE CHANGING GENERATIONS 



The success of the reptiles was based on their early acquisition of a 

 reproductive device which made it possible for them to breed on land. 

 This was the shelled amniote egg — actually a combination of several 

 "inventions," some old and some new. The most important of these are 

 the yolk sac, an embryonic stomach which grows around the food mass 

 stored in the egg and is as old as the fishes; the allantois, a new organ 



Fig. 28.1. The primitive protoreptile Seymouria from the Texas Permian. This creature was 

 less than 3 feet in length. It retained many amphibian characteristics, and is close to the 

 base of the reptilian stem. (Courtesy American Museum of Natural History.) 



which, pressed against the inside of the shell, enables the embryo to 

 respire and also serves for waste storage; the amnion, an enveloping sac 

 that encloses the embryo in a fluid-filled space which is equivalent to 

 the amphibian's pond; the shell, a gas-permeable protective covering 

 secreted around the egg after fertilization; and internal fertilization, 

 occasionally developed in fishes and amphibians but essential for repro- 

 duction on land. Whether or not Seymouria laid shelled eggs is unknown, 

 though as our illustration shows it is often assumed to have done so. The 

 shelled amniote egg is found in all true reptiles and birds and in some 



