Animals that lay eggs to give birth to their young ones are known as oviparous animals. Birds, fishes, amphibians, reptiles and insects are known to show oviparity. Conversely, mammals such as us humans show viviparity, where the growth and nourishment of the embryo take place inside the individual organism.

Now, most of the oviparous animals are females that lay eggs for reproduction. It is considered as an ancestral mode of reproduction. The eggs are fertilised either internally in the body of the females, such as in birds and reptiles.

However, in amphibians, the fertilisation takes place externally after the female has laid the eggs. The animals produce a large number of eggs as compared to the live young produced by viviparous animals. Though, only a limited number of the eggs survive because of maternal care and environmental conditions.

The Egg

• The size and features of eggs vary enormously across genera and species. However, the size is characteristic of each species.
• An important feature for the differentiation of eggs is the deposition of yolk. The process of yolk deposition in the eggs is known as vitellogenesis.

Types of Egg Based on the Amount of Yolk

1. Microlecithal Eggs: The eggs have a very small amount of yolk. E.g., sea urchin
2. Mesolecithal Eggs: The eggs have a moderate amount of yolk. E.g., frogs and toads
3. Macrolecithal Eggs: The eggs have a large amount of yolk. E.g., bony fishes and reptiles

Types of Egg Based on the Position of Yolk

1. Homolecithal Eggs: The yolk is evenly distributed. E.g., annelids and molluscs
2. Telolecithal Eggs: The distribution of yolk is uneven. E.g., birds and fish
3. Meiolecithal Eggs: The yolk makes the most of the egg with a small area for cytoplasm. E.g., egg-laying mammals
4. Centrolecithal Eggs: The yolk is present at the centre of the egg. E.g., insects

Metamorphosis

Metamorphosis is the phenomenon of undergoing abrupt changes in the physical body of animals for growth and development. For example, a butterfly grows into a fully grown insect via four stages; egg → caterpillar → pupa → adult.

At each stage, the insect undergoes vivid changes and develops into a butterfly.

Exception

Seahorses, pipefishes and sea-dragons that belong to the family of Syngnathidae have an exception where the males lay eggs. Seahorses have a pouch where the eggs are developed, whereas pipefishes and sea-dragons carry the developing eggs on the ventral side of their bodies. Learn more about human body parts and other biology-related topics by exploring BYJU’S Biology.

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