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: Concept Overview:
The yolk sac is one of four extraembryonic membranes in amniotes (reptiles, birds, and mammals) that supports the developing embryo. Each membrane performs a specific function.

Step 2: Function Details:
- Yolk Sac: Primarily, it encloses the yolk, the embryo's primary nutrient source. It develops extensive blood vessels to absorb and transport these nutrients to the embryo. In mammals, it's the initial site of blood cell formation, corresponding to option (D).
- Chorion and Allantois: The chorion, located under the eggshell, combines with the allantois to form the chorioallantoic membrane, crucial for gas exchange (option A).
- Allantois: It stores metabolic waste products, notably uric acid (option B).
- Amnion: This membrane contains amniotic fluid, providing a protective cushion against mechanical shock and dehydration (option C).

Step 3: Conclusion:
The yolk sac's main function is nutrient uptake from the yolk and establishing the initial circulatory system for nutrient transport. Therefore, option (D) is correct.