**Core Concept:**
The primitive streak is an embryonic structure that plays a crucial role in the development of the embryonic disc, germ layers, and ultimately, the formation of the body axes in vertebrates. It is initiated and maintained by specific signaling centers, including the node and notochord, which regulate cell differentiation and migration during embryonic development.

**Why the Correct Answer is Right:**
The correct answer is maintained by the interaction between the node and notochord, which are important signaling centers. The node, a group of cells located ventral to the primitive streak, secretes several signaling molecules that guide embryonic development, particularly in the formation of the body axes and the establishment of the notochord. The notochord, a transient embryonic structure that provides structural support and guidance cues, is derived from the notochordal sheath.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because the node is not solely responsible for maintaining the primitive streak. The interaction between the node and notochord is essential for primitive streak maintenance.
B. Although the notochord is involved in embryonic development, it is not the correct answer because the node plays a more significant role in maintaining the primitive streak.
C. This answer is incorrect due to the same reasons as option B. Primitive streak maintenance relies on the interaction between the node and notochord, rather than the notochord alone.
D. This answer is incorrect because the primitive streak is maintained by the interaction between the node and notochord, not just the notochord.

**Clinical Pearl / High-Yield Fact:**
The correct functioning of the node and notochord is essential for proper embryonic development, particularly in establishing the body axes and guiding cell differentiation. Understanding the role of these structures is vital for understanding embryonic development and its potential implications in developmental disorders or malformations.