**Core Concept**
Germ cells in the human embryo originate from the primordial germ cells that migrate from the yolk sac to the gonadal ridges during embryonic development. This process is crucial for the formation of the reproductive system.

**Why the Correct Answer is Right**
The germ cells appear in the yolk sac during the third week of embryonic development, specifically in the yolk sac endoderm. These cells then undergo a process called primordial germ cell migration, where they migrate to the gonadal ridges via the dorsal mesentery. The migration of germ cells is essential for the development of the ovaries or testes. The correct answer is related to the timeline of embryonic development and the specific location of germ cells in the yolk sac.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because germ cells do not appear in the placenta, but rather in the yolk sac.
**Option B:** This option is incorrect because germ cells do not appear in the amnion, which is a membrane surrounding the embryo.
**Option C:** This option is incorrect because germ cells do not appear in the umbilical cord, which is a vital structure for fetal development.

**Clinical Pearl / High-Yield Fact**
The migration of primordial germ cells to the gonadal ridges is a critical event in embryonic development, and any disruptions to this process can lead to abnormalities in reproductive development.

**Correct Answer:** C. 8th week.