13.15: Fertilization
In most fish and amphibians (frogs and toads) fertilization of the egg cells takes place outside the body. The female lays the eggs and then the male deposits his sperm on or at least near them. In reptiles and birds, eggs are fertilized inside the body when the male deposits the sperm inside the egg duct of the female. The egg is then surrounded by a resistant shell, “laid” by the female and the embryo completes its development inside the egg.
In mammals the sperm are placed in the body of the female and the eggs are fertilized internally. They then develop to quite an advanced stage inside the body of the female. When they are born they are fed on milk excreted from the mammary glands and protected by their parents until they become independent.