The most anterior/front-most part of our embryonic disc, before it folds up into a solid/3D body, is destined to become the diaphragm. The next most anterior will become the heart, followed by the mouth, and then the brain. Breath is from our leading edge, our most forward part. Let me explain.

When our ancestors moved on to the land they needed to preserve water. They developed eggs with thick, waterproof, shells and lots and lots of yolk to support that isolated (encapsulated) development.

And this got in the way of the 3D development.

Land animals (with their thick shelled eggs) developed, embryologically, as a disc on the surface of the yolk.

And then this folds - both the front and the sides fold up. They meet, the edges, at the umbilicus.

The front-most part of the embryonic disc, before it folds, is called the septum transversum, and this will become part of the diaphragm. (As it folds and passes backwards under the growing neural tube, it grabs {as it were}, becomes associated with nerves from the neck region which will form the phrenic nerve {C3,4,5}which innervates the diaphragm.)

Next behind this are the cells which will form the heart. (As this folds under the embryo it latches onto the vagus nerve, the tenth cranial nerve.)

Behind this are the cells that will form the mouth.

Behind that, the brain, the nervous system.

I just think this is really neat, that the foremost part of us is associated with our breath. Then the heart. Only then the mouth and the brain.