In the egg of a bird, the embryo is located in the

Analyze the question: The question asks where the embryo (the developing chick) is located in a bird's egg.

Explain the concept: A bird's egg has these main parts:

• Shell and shell membranes: protect the egg and allow gas exchange.
• Air space (air cell): a pocket of air at the blunt end, which supplies air to the chick just before hatching.
• Albumen (egg white): provides water and protein, and cushions the egg.
• Chalazae: twisted cords of protein that hold the yolk in the centre of the albumen.
• Yolk: the food store (fats, proteins). On its surface sits the germinal disc (blastodisc), a small whitish spot which is where the embryo develops if the egg is fertilised.

Step-by-step solution:

1. The chalaza only suspends the yolk, so it does not contain the embryo. This eliminates A.
2. The albumen is a source of water and protein and does not house the embryo. This eliminates C.
3. The air space is for respiration and does not house the embryo. This eliminates D.
4. The embryo develops from the germinal disc, which lies on the surface of the yolk. The embryo is therefore located in or on the yolk, and it feeds on the yolk as it grows.

Answer: B. Yolk

Exam tip: Remember that the embryo sits on the yolk, and that the yolk is its food. For the other parts, think "Chalaza = Chain (holds the yolk), Albumen = Water, Air space = Air."