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In telolecithal egg, the yolk is found in:
A. All over the egg
B. On one side
C. Both the sides
D. In the centre

Last updated date: 21st Jun 2024
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Hint: Eggs are laid by the female members of many different species of organisms including the birds, reptiles, fishes, amphibians as well as lower animals like insects. Some mammalian species are also known to lay eggs. The egg contains a protective shell, known as the eggshell and contains the vitellus or the egg yolk and the albumin or the egg white.

Complete answer:
The egg yolk is the major and significant portion of the egg as it contains important proteins and choline which are necessary for the growing embryo. The egg yolk is suspended in the portion of the egg white in the form of one or even two spiral bands of tissue that is known as the chalazae.

The eggs are classified into three types namely, isolecithal, telolecithal and centrolecithal on the basis of distribution and amount of yolk.

If yolk is found all over the egg that is on both sides of the egg then the egg is known as isolecithal egg. Usually, isolecithal refers to the fair distribution of the yolk in the ova cytoplasm of mammals and other vertebrates, in particular the fish of the Petromyzontidae, Amiidae and Lepisosteidae families. Isolecithal cells have two identical hemispheres of yolk.

In some eggs, yolk is partially distributed on one side of the side, the egg known as telolecithal eggs. It refers to the uneven and unequal distribution of yolk in the ova cytoplasm and is found in birds, reptiles, fish and monotremes. The yolk is concentrated in a single pole of an egg separate from a developing embryo. They are also known as macrolecithal eggs.

The yolk is placed in the center of the cytoplasm of ova in case of a centrolecithal egg. A lot of arthropod eggs are centrolecithal. During the process of cytokinesis, the centrolecithal zygotes undergo meroblastic cleavage. In this process the cleavage plane only extends to the accumulated yolk and is superficial. Telolecithal eggs contain moderate or large quantities of yolk. Also, the distribution of yolk is not uniform and found to be distributed unevenly.

Hence, The correct answer is option (B).

Additional information:
Telolecithal eggs can be further divided into three types:
> Slightly Telolecithal: This type of egg contains only a small amount of yolk which is scattered unevenly. The vegetal pole has the largest concentration and the lowest animal pole (e.g. fish eggs).

> Moderately Telolecithal egg: This form of egg contains a moderate amount of yolk that is distributed unequally. Due to the presence of high concentration of yolk in the vegetal hemisphere, the nucleus is moved more towards the animal hemisphere (e.g. the egg of amphibians).
> Extremely Telolecithal Egg: In this type of egg, due to the strong deposition of the yolk, the entire vegetal hemisphere and the main portion of the animal hemisphere is occupied by the yolk. Due to the nature of unequal distribution of the yolk, the ooplasm and the nucleus are shifted towards the animal pole (e.g. reptilian and avian eggs).

Note: The telolecithal eggs are the eggs in which the yolk is unevenly distributed in the cytoplasm of. In the telolecithal eggs, the yolk is more abundant in one hemisphere of eggs than the other. They mostly occur in fish, amphibians, reptiles, and birds. The yolk is deposited on the vegetal pole and supports the embryo development. The yolk is not included in the cells during cell division and this type of eggs undergo discoidal meroblastic cleavage.