Internal & External Fertilisation

Extracted from HERE:

Internal fertilisation occurs when the male deposits sperm directly into the female's body. Most land animals, including insects, reptiles, birds, and mammals, reproduce in this way. In almost all mammals and some reptiles, the embryo develops inside the female's body after fertilisation. However, in birds and most reptiles, the embryo develops outside her body. The female lays an egg in which the embryo develops. The egg is protected by a shell and contains material that nourishes the growing embryo.

Among animals, fertilisation may be external or internal. During external fertilisation, male and female sex cells unite outside the female's body. This process occurs in water. The male releases sperm into the water at about the same time that the female releases eggs. Some sperm come into contact with eggs and fertilise them. Almost all aquatic invertebrates and some vertebrates, including the majority of fish and amphibians, reproduce in this way.

The eggs of most fish are fertilised outside the female's body. A female releases her eggs into the water at the same time that a male releases his sperm. Some sperm come in contact with some of the eggs, and fertilisation takes place. The entire process during which eggs and sperm are released into the water and the eggs are fertilised is called spawning. Almost all bony fish reproduce in this way.

Sharks, rays, chimaeras, and a few bony fish, such as guppies and mosquito fish, reproduce in a different manner. The eggs of these fish are fertilised inside the female. For internal fertilisation to occur, males and females must mate. After fertilisation, the females of some species release their eggs into the water before they hatch. Other females hatch the eggs inside their bodies and so give birth to living young. Fish that bear living young include many sharks, rays, and guppies.