Which of the following is not true about chick embryology?

Question

Accepted Answer

The correct option is 1: The Eggshell is formed of Silica.

This statement is **not true** and is the correct answer to the question. The chemical composition of the avian eggshell is a well-established biological fact. The eggshell is composed overwhelmingly of **Calcium Carbonate (CaCO3)**, which typically makes up about 95% or more of its dry weight. This mineral compound is what gives the eggshell its hard, rigid structure, forming a crucial protective barrier for the embryo and its contents. Silica, or Silicon Dioxide (SiO2), is a completely distinct substance and is not the primary constituent of the eggshell. Therefore, stating that the eggshell is formed of Silica is factually incorrect regarding chick embryology.

The incorrect option is 2: Cleavage in chick embryo is meroblastic.

This statement is \*\* true\*\* about chick embryology. The chick egg is a classic example of a **telolecithal** egg, meaning it contains a very large amount of densely packed yolk. Due to this massive yolk quantity, the cleavage furrow, which is the indentation that divides a cell, is physically prevented from cutting through the entire egg. Consequently, cell division is restricted to a small, disc-shaped area of cytoplasm located on top of the yolk, known as the blastodisc. This incomplete and partial pattern of cell division is precisely what is termed **meroblastic cleavage**.

The incorrect option is 3: Chick embryo receives its nutrition from yolk sac.

This statement is **true** and highlights the primary role of an extra-embryonic membrane. The chick embryo is a **cleidoic** egg, meaning its development occurs entirely outside the mother's body, and thus it must rely on the stored resources within the egg. The **yolk sac** is the membrane that grows to enclose the yolk mass. Its epithelial cells are specialized for digesting, absorbing, and transporting the rich nutrients—including proteins, fats, and vitamins—from the yolk into the embryo's circulatory system. This mechanism is essential for the growth and survival of the chick embryo throughout its incubation period.

The incorrect option is 4: Eggshell membranes are two in number.

This statement is also **true**. Directly beneath the outer calcareous shell layer, a chicken egg is protected by two distinct layers of protein fiber: the **outer shell membrane** and the **inner shell membrane**. These two transparent membranes are surprisingly strong and function as an important secondary line of defense against microbial contamination. They are typically found lying closely together, but at the larger end of the egg, they separate to enclose a small pocket of air, which is known as the air cell.