At the end of week 2 of development (day 14), what is the composition of the embryonic disk?
**Core Concept**
The embryonic disk at the end of week 2 of development (day 14) is a critical stage in embryogenesis, characterized by the formation of the epiblast and hypoblast layers. The epiblast layer will eventually give rise to the three primary germ layers: ectoderm, mesoderm, and endoderm. The hypoblast layer plays a crucial role in the formation of the yolk sac.
**Why the Correct Answer is Right**
At the end of week 2 of development, the embryonic disk consists of two distinct layers: the epiblast and the hypoblast. The epiblast is a thick, compact layer of cells that will eventually give rise to the three primary germ layers through a process called gastrulation. The hypoblast layer, on the other hand, is a thinner layer of cells that will contribute to the formation of the yolk sac. The hypoblast layer also plays a role in the formation of the primitive streak, a linear structure that will eventually give rise to the notochord, somites, and other embryonic structures.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the composition of the embryonic disk at the end of week 2 of development. While the epiblast and hypoblast are indeed present, they are not the only components of the embryonic disk.
* **Option B:** This option is incorrect because it is too vague and does not accurately describe the specific layers present in the embryonic disk at this stage.
* **Option C:** This option is incorrect because it is not accurate. While the epiblast and hypoblast are present, they are not the only components of the embryonic disk, and the primitive streak is not yet formed.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the embryonic disk at the end of week 2 of development is a critical stage in embryogenesis, and the formation of the epiblast and hypoblast layers is a key milestone in the development of the three primary germ layers.
**Correct Answer:** C. Epiblast and hypoblast.