Question

Eggs of egg laying mammals are
a. Macrolecithal
b. Alecithal
c. Mesolecithal
d. All of the above

Answer 1

The eggs are classified in the basis of amount and distribution of yolk in them. Yolk is the reserve food material in the cytoplasm of eggs. Yolk is formed by phospholipids, proteins, lipids and carbohydrates.

Complete answer:
Macrolecithal or megalecithal are the eggs of egg laying animals. The eggs contain a very large amount of yolk, e.g., reptiles and birds.
The process of formation of yolk and its deposition in egg is called vitellogenesis. The eggs are classified on the basis of amount and distribution of yolk in them.

1. Based on the amount of yolk:
• Alecithal: Eggs without yolk. e.g., human egg.
• Microlecithal: Eggs with very little yolk, e.g., sea urchin and starfish.
• Mesolecithal: Eggs containing moderate amounts of yolk, e.g., frog.
• Macrolecithal: Eggs containing very large amounts of yolk, e.g., reptiles and birds.

2. Based in the distribution of yolk:
• Isolecithal: The yolk is uniformly distributed throughout the cytoplasm of egg, e.g., Branchiostoma.
• Telolecithal: The yolk is concentrated towards one pole, e.g., amphibians.
• Centrolecithal: The yolk is concentrated in the centre of the egg, e.g., insects.

There are three type of membrane around the eggs:
• Primary egg membrane: It is formed around the plasma membrane of the egg and produced by the ovum itself.
• Secondary egg membrane: It is formed around the egg by the follicle cells of the ovary.
• Tertiary egg membrane: This membrane is formed by the oviducts and other parts of the maternal genital tract when the egg is passing from the ovary to the exterior.

Hence, the correct answer is option (A).

Note: The primitive egg laying mammals like platypus also lays macrolecithal. Cleidoic eggs are the eggs of reptiles and birds which are insulated from the environment by albumen, membranes and shell.