Hypoblast forms:
Wait, but the question is about what the hypoblast forms. The yolk sac is a key structure here. The yolk sac is important for early blood cell formation and is derived from the hypoblast. The epiblast, on the other hand, gives rise to the three germ layers through gastrulation.
So the correct answer should be the yolk sac. Let me check the options again. The user didn't list the options, but the correct answer is likely the yolk sac. The other options might be structures like the amnion, chorion, or allantois. The amnion comes from the epiblast, the chorion from the trophoblast, and the allantois from the epiblast as well. So the hypoblast forms the yolk sac. That's the core concept here.
**Core Concept** The hypoblast is one of the two primary layers formed during the differentiation of the inner cell mass of the blastocyst. It gives rise to the yolk sac, a structure critical for early hematopoiesis and nutrient exchange before placental function is established.
**Why the Correct Answer is Right** The hypoblast directly forms the **yolk sac**, which develops from the hypoblast lining the blastocyst cavity. The yolk sac contributes to the formation of the primitive gut and is the site of yolk sac hematopoiesis, producing erythroblasts in early embryonic life. It also plays a role in the formation of the umbilical vesicle and contributes to the germ cells.
**Why Each Wrong Option is Incorrect**
**Option A:** The amnion is derived from the epiblast, not the hypoblast.
**Option B:** The chorion originates from the trophoblast, not the hypoblast.
**Option C:** The allantois develops from the epiblast during gastrulation, not the hypoblast.
**Clinical Pearl / High-Yield Fact** Remember the mnemonic **"HYPO-Yolk"** to recall that the hypoblast forms the yolk sac. The yolk sac is a critical structure for early development and is often assessed in prenatal ultrasounds as a sign of a viable pregnancy.
**Correct Answer: C. Yolk sac**