**Core Concept**
The notochord is a precursor to the vertebral column and plays a crucial role in the development of the central nervous system. It is a flexible, rod-like structure that forms in the embryonic mesoderm during the third week of gestation.

**Why the Correct Answer is Right**
The notochord develops from the mesoderm, specifically the axial mesoderm, which is a group of cells that runs along the midline of the embryo. The axial mesoderm eventually differentiates into the notochord, a structure that provides axial stiffness and support to the embryo. The notochord also serves as a signaling center, inducing the formation of the neural plate and the development of the central nervous system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the notochord does not develop from the ectoderm, which is the outermost layer of the embryo.
**Option B:** This option is incorrect because the notochord does not develop from the endoderm, which is the innermost layer of the embryo.
**Option C:** This option is incorrect because the notochord does not develop from the somatic mesoderm, which gives rise to the somites and eventually the skeletal muscles.

**Clinical Pearl / High-Yield Fact**
The notochord is a key structure in the development of the vertebral column and the central nervous system. Its failure to develop properly can result in congenital anomalies such as spina bifida or anencephaly.

**Correct Answer:** D.