Why do fish and frogs lay eggs in hundreds whereas a hen lays only one egg at a time?

In animals such as frogs and fishes, the process of fertilization takes place outside the body. This type of fertilization is known as external fertilization.

Frogs and fish mostly lay their eggs in water; larvae produced by them could flow along with water so to protect their generation they have to lay thousands of eggs at a time while hens lay eggs on land, so they do not need to lay many eggs.

Organisms having internal fertilization, like hen, produce one egg at a time. This is so because here there is less chance of failure of fusion between male and female gamete. After the gametes are fertilized there is just the need of development of animal in the form of egg. Frog like aquatic organisms who shows external mode of reproduction have to lay egg in hundred. The first reason behind this is the single egg is so small in size that it will not be possible to stay to definite position and which in turns make hard for sperm to found and fuse them. Whereas, when large number of eggs make them stick together this makes fertilization possible. Another reason behind this is there is a lot of organisms who feed on these eggs, so larger number of eggs gives more possibility of giving birth to young ones.

As the egg of hen is in its uterus, so when the egg is formed it instead of taking birth goes direct to the opposite of fallopian tube and many protective layers are formed. The hard shell is one such layer. Hen isn’t an oviparous animal but a viviparous one.