**Core Concept**
Primordial germ cells (PGCs) are the earliest precursors of the gametes in the developing embryo. They are the origin of the germ cell lineage, which eventually gives rise to the gametes, or sex cells. The development of PGCs is a critical process in the formation of the reproductive system.

**Why the Correct Answer is Right**
Primordial germ cells develop from the epiblast, specifically from a region called the primitive streak in the early embryo. The epiblast is one of the two layers of cells formed during gastrulation, a process of embryonic development. The primitive streak is a linear array of cells that forms in the epiblast and serves as a site of cell migration and differentiation. PGCs migrate from the primitive streak to the yolk sac, where they undergo proliferation and differentiation to form the primordial germ cell population.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the mesoderm is one of the three primary germ layers, but it is not the source of primordial germ cells. The mesoderm gives rise to connective tissue, muscle, and other structures.
**Option B:** This option is incorrect because the endoderm is another of the three primary germ layers, but it is not the source of primordial germ cells. The endoderm gives rise to the lining of the digestive tract, respiratory system, and other internal organs.
**Option C:** This option is incorrect because the ectoderm is the outermost of the three primary germ layers, but it is not the source of primordial germ cells. The ectoderm gives rise to the nervous system, skin, and other external structures.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the development of primordial germ cells is a critical process in the formation of the reproductive system, and any disruptions to this process can lead to reproductive abnormalities or infertility.

**Correct Answer:** D.