While it's not a traditional organ in the sense of having a specific function, the primitive streak is a crucial structure that sets the stage for the development of all other organs. It appears on the dorsal surface of the embryo and establishes the body's main axes (head-to-tail and left-to-right).

It's important to note that the primitive streak doesn't have a specific function itself, but it's the starting point for:

* Gastrulation: The process where the single-layered blastula (the early embryo) transforms into a three-layered gastrula, with ectoderm, mesoderm, and endoderm. These layers are the foundation for all the organs and tissues in the body.

* Formation of the neural tube: The precursor to the brain and spinal cord, which begins to develop from the ectoderm layer.

So, while the primitive streak isn't an organ in the traditional sense, it's the first essential structure to appear in the embryo and lays the groundwork for all the other organs to develop.