A germ layer is a layer of cells during fetal development that all have similar fates. Germ layers are only really pronounced in the vertebrates, but present in all eumetazoans. According to the number of layers animals can be termed diploblastic (with two) or triploblastic (three layers). All eumetazoans except for those in the branch radiata, are triploblastic.

Together, the three layers are called the primary germ layers, since they originate all organs in the body.

The mesoderm layer will form the muscles, many organs, connective tissue, and bones. The ectoderm layer will form the epidermis and nervous system, while the endoderm will produce the gastrointestinal tract, respiratory tract, and endocrine glands.