The neural crest at the time of its formation is represented by a mass of loose cells lying dorsal to the neural tube. Almost at once after the formation of the crest, the cells of which it consists start migrating in a lateral and ventral direction from the place of their origin.
As the neural crest cells move, they form streams, bypassing, as they go, certain organs (viz., the eye, and the gill pouches). These streams of neural crest cells are especially conspicuous in the head and neck region, while in the trunk region the neural crest cells are more scattered right from the start.
Some of the neural crest cells move into the space between the epidermis and the layer of mesoderm; others penetrate into the interstices between the neural tube and the inner surface of the somites and down to the dorsal aorta and beyond. In their movements the neural crest cells follow mesenchyme-filled spaces between organ rudiments.
Sooner or later, the most advanced neural crest cells reach the mid-ventral line of the body. Not all of them,