The first organ to develop in an embryo is the primitive streak. It's not quite an organ in the traditional sense, but it's a crucial structure that establishes the body axis and sets the stage for the formation of all other organs.

Here's why the primitive streak is considered the first organ:

* It's the first structure to appear on the dorsal surface of the embryo. This occurs around 14 days after fertilization.

* It establishes the body's main axis. The primitive streak defines the head-to-tail direction and the left and right sides of the developing embryo.

* It initiates gastrulation. This is a critical process where the single-layered embryo transforms into a three-layered embryo (ectoderm, mesoderm, and endoderm) which will eventually give rise to all of the body's organs.

While the primitive streak itself isn't a functional organ, it's essential for the formation of all other organs. It's the foundational structure for a developing human.