**Question:** Primitive streak develops from

A. Isthmus
B. Ectoderm
C. Ectoplacental cone
D. Mesoderm

**Core Concept:**
In the developing embryo, the primitive streak is a key structure that leads to the formation of the embryonic disc and subsequently forms the body axes, notochord, and primitive streak. The primitive streak plays a crucial role in the process of gastrulation.

**Why the Correct Answer is Right:**
The correct answer, C. Ectoplacental cone, refers to the structure that gives rise to the primitive streak during the early stages of embryonic development. The ectoplacental cone is composed of three layers: ectoderm, mesoderm, and endoderm. The primitive streak develops from the ectoderm layer of the ectoplacental cone, which is essential for gastrulation and the formation of the embryonic disc.

**Why Each Wrong Option is Incorrect:**

A. Isthmus (option A) is a narrow band of cells connecting the coelom and gut, and is not involved in the formation of the primitive streak.
B. Ectoderm (option B) is one of the three germ layers of the ectoplacental cone but does not form the primitive streak directly.
D. Mesoderm (option D) is a germ layer but is not the source of the primitive streak; rather, it develops from the endoderm layer of the ectoplacental cone.

**Clinical Pearl:**
Understanding the development of the primitive streak is crucial for understanding embryonic development, as it leads to the formation of the body axes, notochord, and primitive streak, which are essential for further organogenesis and differentiation of tissues in the developing embryo. A deep understanding of germ layers and their differentiation is essential for medical students, especially in the context of embryology and developmental biology.