Why must reptiles and birds use internal fertilization?
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Reptiles were the first fully terrestrial vertebrates to evolve on Earth. One adaptation that allowed them to live a terrestrial life was the evolution of the amniotic egg and internal fertilization. With internal fertilization, there is a greater chance of the sperm to be delivered to the egg and for fertilization to occur. This is in contrast with Amphibians, which have external fertilization in a freshwater environment. There is a risk that many of the eggs will never be fertilized by sperm and thus, thousands are released to help insure some become fertilized. However, after the female reptile or bird lays the fertilized eggs, they are equipped with various membranes to support the embryo. These include the outer porous shell and the chorion which is a moist respiratory membrane beneath the shell for gas exchange. There is the allantois, to store wastes, the yolk sac to provide food and an amnion for the embryo to develop in. All of these adaptations made reptiles and later on, the birds able to successfully reproduce on land.
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