An egg cell is a single cell. When fertilized, it is called a zygote. Soon the zygote develops, increasing in number of cells and starts forming organs. At this stage, it is called embryo.
The female fish releases many egg cells in the water. Then the male fish also releases a great number of sperm cells. When an egg cell and sperm cell meet, fertilization takes place in the water. The female frog releases many egg cells in the water. The egg cells are surrounded by a jellylike material. Soon, the water causes this material to swell and become frothy like beaten egg white. As the egg cells come out of the female, the male frog releases sperm cells over them. Fertilization takes place. As in fish, fertilization in frogs takes place outside the body of the female frog in the water. In frogs, however the egg cells are held together by the frothy material.
Fertilization in fish and frogs is described as external fertilization. Since the fertilized eggs of these two animals develop outside, they also have external development of the embryo.
Now, let's take a look at cats. In cat, fertilization of the egg cells as well as development of the embryo is internal. They take place inside the body of the female.
Let us differentiate the egg cell from the egg of birds. The egg usually contains a fertilized egg cell which the female lays to continue the development of the embryo outside its body. It is generally covered by a shell that serves two purposes. It prevents loss of water from the egg. And it also protects the egg from physical damage, including attack by predators. Birds, turtles, crocodile and other animals that lay eggs have internal fertilization. But the fertilized eggs, which are protected by shell, develop outside the body of the female. The female bird sits on the eggs to incubate them. The heat from the bird's body hastens the development of the embryo until it is ready for hatching. But how about turtles? Where does the heat that incubates the eggs come from? The sun warms the sand where the turtle's eggs are buried.
Some animals like birds and most snakes lay eggs with the embryo inside. The embryo develops outside the body of the female. The egg hatches when the young animal has used up the food supply (the yolk) inside the egg and can already survive outside the shell. Animals that lay eggs are said to be oviparous.
On the other hand, animals liked dogs, cats, horses and others, develop their embryo inside the mother's body and give birth to their live young. They are said to be viviparous.
note: originally posted at Exposeknowledge.com under the same author.