Based on the retrieved information and standard embryology knowledge, the evaluation of the statements about extraembryonic structures in mammals is as follows:

A. Allantois (not “allantoin”) stores urinary waste and helps mediates gas exchange. It is derived from splanchnopleure at the caudal end of the primitive streak. This statement is correct, except there is a typo in the question: the correct term is allantois, not allantoin.

B. Amnion is a water sac that protects the embryo and contains amniotic fluid. This membrane is derived from somatopleure. This statement is correct.

C. Chorion is essential for gas exchange in amniote embryos. It is formed mainly from the somatopleure (outer layer) along with the extraembryonic mesoderm. Therefore, the statement that it is generated from splanchnopleure is incorrect.

D. Yolk sac is one of the first embryonic membranes to form, not the last, and it is derived from the splanchnopleure (endoderm + extraembryonic mesoderm), not the somatopleure. Hence, this statement is incorrect.

Correct Statements:

• A (Allantois derived from splanchnopleure, stores waste, gas exchange) correct

• B (Amnion derived from somatopleure, water sac) correct

Final Answer:
(1) A and B

Introduction

In mammalian development, extraembryonic membranes play crucial roles in supporting the embryo’s growth and survival. These membranes originate from distinct embryonic layers and perform key functions such as protection, gas exchange, nutrient provision, and waste storage.

The Allantois: Waste Storage and Gas Exchange

• The allantois arises from the splanchnopleure at the caudal end of the primitive streak.

• It collects nitrogenous wastes and facilitates gas exchange by fusing with the chorion to form the chorio-allantoic placenta.

• The allantois is vital for establishing fetal-maternal circulation.

The Amnion: Protective Water Sac

• The amnion is derived from the somatopleure, forming a fluid-filled sac around the embryo.

• This sac cushions the embryo and maintains a stable aqueous environment essential for development.

• The amniotic fluid protects from mechanical shocks and prevents desiccation.

The Chorion: Outer Membrane for Gas Exchange

• The chorion develops primarily from the somatopleure (ectoderm + extraembryonic mesoderm), not the splanchnopleure.

• It contributes to gas exchange between the embryo and mother by forming part of the placenta.

The Yolk Sac: Early Nutrient and Blood Formation Site

• The yolk sac develops from the splanchnopleure early in development.

• Contrary to the statement in question, the yolk sac is one of the earliest membranes to form and plays a role in nutrient transfer and hematopoiesis during early embryogenesis.

Conclusion

In mammalian embryos, extraembryonic membranes derive from two tissues: the somatopleure and splanchnopleure. The allantois (A) and amnion (B) statements are accurate regarding their origin and functions, while the statements about the chorion (C) and yolk sac (D) contain inaccuracies.

Final Answer:
(1) A and B