**Core Concept**
The development of the gut tube is a complex process involving the formation of the embryonic endoderm, which gives rise to the lining of the digestive system. The gut tube is formed through a series of highly regulated and coordinated events, including the invagination of the endoderm and the formation of the foregut, midgut, and hindgut.

**Why the Correct Answer is Right**
The correct answer is **ectoderm** is incorrect, the gut tube is actually formed from the endoderm germ layer. The endoderm is one of the three primary germ layers that form during embryogenesis, the others being the ectoderm and mesoderm. The endoderm gives rise to the lining of the digestive system, including the gut tube, which is composed of epithelial cells that form the lining of the esophagus, stomach, small intestine, and large intestine. The development of the gut tube is critical for the proper functioning of the digestive system.

**Why Each Wrong Option is Incorrect**
**Option A:** Ectoderm is incorrect because the ectoderm gives rise to the nervous system, skin, and other external tissues, not the lining of the digestive system.
**Option B:** Mesoderm is incorrect because the mesoderm gives rise to the muscular, skeletal, and circulatory systems, among others, but not the lining of the digestive system.

**Clinical Pearl / High-Yield Fact**
The development of the gut tube is a complex process that involves the coordinated action of multiple genetic and environmental factors. Understanding the embryology of the gut tube is important for understanding the development of congenital anomalies and abnormalities of the digestive system.

**Correct Answer: C. Endoderm**