## Core Concept
The cornea is the transparent, avascular part of the eye that covers the front portion of the eyeball. It is composed of several layers, with the stroma being the thickest layer, making up about 90% of the cornea's thickness. The corneal stroma is primarily derived from mesenchymal cells.

## Why the Correct Answer is Right
The corneal stroma is derived from **mesenchyme**, which is a type of undifferentiated loose connective tissue that can develop into various connective tissue types, including bone, cartilage, and smooth muscle. The mesenchymal cells migrate into the space between the two layers of the corneal epithelium (the outer layer) and the innermost layer, the endothelium, to form the stroma. This process involves the production of a highly organized extracellular matrix by keratocytes (stromal cells), which provides the cornea with its transparency and strength.

## Why Each Wrong Option is Incorrect
- **Option A:** Ectoderm is incorrect because, while the corneal epithelium is derived from ectoderm (specifically, surface ectoderm), the stroma itself does not originate from ectoderm.
- **Option B:** Endoderm is incorrect because the endoderm gives rise to the lining of the digestive tract, respiratory tract, and other internal organs, not the cornea or its stroma.
- **Option C:** Neuroectoderm is incorrect because, although the eye itself has origins related to neuroectoderm (such as the retina), the corneal stroma does not derive from neuroectoderm.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the cornea's transparency and avascularity are crucial for its function in vision. Any damage to the cornea, such as that caused by infection or trauma, can lead to vision impairment. The cornea's unique structure, including its stromal composition, is essential for maintaining its clarity.

## Correct Answer Line
**Correct Answer: D. Mesenchyme.**