Two main layers of cells comprise the CR. The first one is a condensation of the mesenchyme composed of loosely packed, unspecialized cells derived from the mesoderm. The mesoderm is the midlle of the three primary germ cells layers in embryos. The second layer of the CR is composed of epithelia organized in sheets and tubes of connected cells. The CR forms through a series of reciprocal interactions between the two layers. Such interactions are called epithelial-mesenchymal interactions. The interactions represent a developmental process, which also determine the formation of many structures in the development of vertebrate morphology, such as limbs.