Question

What is the function of yolk sac?

• A. It contains blood vessels that exchange gases with the outside environment.
• B. It stores waste products.
• C. It enables the embryo to float in a fluid environment that protects it from desiccation.
• D. It enables nutrient uptake and the development of the circulatory system.

Hint:

Remember the 'A, B, C' of extraembryonic membranes:

\textbf{A}mnion = \textbf{A}quatic environment (protection).
\textbf{B}ladder (Allantois) = Stores waste.
\textbf{C}horion = Gas exchange (\textbf{C}overing).
Yolk Sac = Food source. \end{itemize}

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The yolk sac is one of the four extraembryonic membranes that support the developing embryo in amniotes (reptiles, birds, mammals). Each membrane has a specialized function.

Step 2: Detailed Explanation of Functions:
- Yolk Sac: Its primary role is to enclose the yolk, which serves as the main source of nutrition for the embryo. It develops an extensive network of blood vessels to absorb these nutrients and transport them to the embryo. In mammals, it is the first site of blood cell formation. This matches option (D).
- Chorion and Allantois: The chorion lies just beneath the eggshell and, along with the allantois, forms the chorioallantoic membrane, which is the main site for gas exchange (option A).
- Allantois: This sac stores metabolic wastes, particularly uric acid (option B).
- Amnion: This membrane encloses the amniotic fluid, which provides a protective, fluid-filled cushion for the embryo, preventing mechanical shock and desiccation (option C).

Step 3: Final Answer:
The principal function of the yolk sac is to facilitate the uptake of nutrients from the yolk and to establish the initial circulatory system to transport these nutrients. Therefore, option (D) is the correct answer.