A germ layer is a group of cells in an embryo that develops into specific tissues and organs. During early development, three primary germ layers form: ectoderm, mesoderm, and endoderm. Each layer has distinct roles; for example, the ectoderm becomes the skin and nervous system, while the mesoderm forms muscles and bones, and the endoderm develops into internal organs. These germ layers arise during a process called gastrulation, where the single-layered blastula transforms into a multi-layered structure. Understanding germ layers is crucial in fields like embryology and developmental biology, as they provide insights into how complex organisms develop from a single fertilized egg.