Historic Background of Reptiles (Part 2)

The reptiles came from amphibian ancestors. The egg of the usual amphibian is almost naked, enclosed only by a jelly envelope. The jelly supports each egg separately in the mass, keeps out small invaders and discourages predation by larger animals, but it gives almost no protection against drying up. A typical frog egg on land on a clear day will quickly wither. Thus, no matter how far the adult frog may be able to move from water in the course of its own daily activities, when the time comes to provide new frogs most species have to go home to the water. The songs of male frogs all over the world calling the females to the ponds show how strong the obligation is.

The egg as the reptiles developed it--which was essentially as we know it today--had no such limitation. Its smooth shell tightly enveloped white and yolk. Like any egg, as it incubated it got more complex inside, and the complexity was not just in the forming body of the new animal but also in the structures required to keep the embryo alive in its shell-- to keep it supported, fed, and neither poisoned nor asphyxiated. The structures that did this are known as the embryonic membranes. The reptiles evolved them and kept by the birds; and, with modifications, they also serve as embryonic structures of the mammals. Because they occur both in the shelled egg and in the uterine development of the mammal, all three higher vertebrate classes-- mammals, reptiles and birds-- are collectively called amniotes. The name refers to one of the embryonic membranes, the amnion, which shuts in a fluid in which the embryo is able to go on leading an essentially aquatic existence as it develops. A yolk sac is held by a stalk from the belly region of the embryo, and just behind its attachment is that of the allantois, another sac which partly fills the space between the amnion and a third membrane, the chorion, which lies just beneath the shell. The allantois receives and stores embryonic waste, serving as a sort of bladder. It also has blood vessels that pick up oxygen that passes through the shell and conduct it to the embryo. The shell reduces evaporation, but it is porous and does not wall the embryo in completely. It shuts out prying small animals, for example, but not the oxygen the embryo requires to live.

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