Question

Microlecithal eggs have
A. No yolk at all
B. A small amount of yolk
C. A large amount of yolk
D. A large amount of yolk concentrated at one pole

Answer 1

Eggs are usually classified into two types of classifications. These are based on-
- The quantity of yolk in eggs
- The distribution of yolk in cytoplasm.
The Yolk of the egg is used for two purposes—supply of energy and synthesis of the products that are required for the elaboration of the embryonic body. The yolk also plays a role in following
- The size of the ovum,
- Differentiation of ooplasm,
- Patterns of cleavage,
- Morphogenetic movements of blastomeres during gastrulation,
- The type of development whether direct or indirect.

Complete answer:
Eggs classified on the basis of the yolk-

Microlecithal-
Small eggs with a small amount of yolk. The yolk is evenly distributed. When we leave these cells, they cut through and divide them into cells of fairly similar sizes. Example- The eggs of Amphioxus, marsupials, eutherian mammals, etc.

Mesolecithal-
In these eggs, the yolk is moderate in amount. The yolk is distributed distinctly unequal. Example- The eggs of sharks, ganoid fishes, Dipnoi, and many amphibians.

Macrolecithal-
In these cells, an Enormous amount of yolk is present and here yolk is several times greater than the cytoplasm. These eggs can be small or big. Example- The eggs of teleost fishes, Gymnophiona (legless amphibians), reptiles, birds, and monotremes (egg-laying mammals).

- Option A - can not be right because cells with no yolk are known as alecithal eggs.
- Option B - a small amount of yolk is present in microlecithal eggs.
- OptionC - cells with a large amount of yolk are known as macrolecithal eggs.
- Option D - In microlecithal eggs, the yolk is always evenly distributed.

So the right answer is option B i.e, a small amount of yolk.

Note: Mammalian (especially placental) egg cells also have very little yolk, we can consider them as microlecithal eggs but as the amount of yolk is negligible, they are considered as alecithal eggs. The microlecithal eggs are small in size and the amount of yolk is much less than the amount of cytoplasm.