Final answer:
The neural crest is called the "fourth germ layer" because it is a critical group of cells originating from the neural plate region during neurulation, forming various tissues in both the nervous system and other parts of the body.
Step-by-step explanation:
The neural crest is often referred to as the "fourth germ layer" because it is a unique population of cells that originate from the border of the neural plate during the complex process of embryonic development known as neurulation. Once the ectoderm differentiates into a neuroectoderm and forms the neural plate, neural plate borders called neural folds elevate and eventually fuse to create the neural tube. As the neural tube closes, the neural crest cells separate from the ectoderm and disperse throughout the embryo to give rise to various tissues within the peripheral nervous system (PNS), such as neurons and glia, as well as non-neural tissues like craniofacial cartilage, bone, and melanocytes. These cells are so influential in embryo development and contribute to such a diverse array of tissues that they are sometimes given a distinction akin to the primary germ layers: ectoderm, mesoderm, and endoderm.