There’s predatory pressure in the sea, so some of the shallow-water vertebrates, seeking food and refuge ashore, gradually acquired legs, lungs, scales and the shelled egg, and thus developed as last into land reptiles which were able to forage for insects in the forest.
The process, of course, was not quite that flowing and easily traced. There is a good deal of controversy among experts about the order in which the separate innovations appeared and what immediate selective advantage was gained in each case. One fact is clear, however. Of all the adaptations that fitted the vertebrates into their increasingly refined roles on land, none was more fundamental than the reptilian egg.
Most people, thinking of an egg, think of a bird; but they are being led astray by seeing eggs mainly at breakfast. The birds did not invent the shelled egg, they inherited it, and it has undergone no important evolution in their possession. The first shelled land eggs were reptilian, and the reptiles were reptiles only when they had evolved such an egg. The old riddle, “which came first, the hen or the egg,” is just whimsey when the hen is a bird. But applied to reptiles the question is valid, and paleontologists are still getting testy with each other trying to answer it.
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 it comes time to provide new frogs most species have to go home to the water. The sons 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 shut in 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, unpoisoned and unasphyxiated.
The structures that did this are known as the embryonic membranes. They were evolved by the reptiles 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 stalked 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 cuts down evaporation, but it is porous and does not wall the embryo off completely. It shuts out prying small animals, for example, but not the oxygen the embryo requires to live. For the embryo to thrive, such an egg must be kept warm and not too dry.
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