The eardrum forms from the joining of the expanding first pharyngeal pouch and groove.Around day 30 of gestation, the endoderm-lined first expands to form the tympanic cavity, which subsequently envelops the inner ear ossicles.Simultaneously, the first pharyngeal groove, which is lined with ectoderm, expands to form the developing external auditory meatus.Separated by a thin layer of splanchnic mesoderm, the tympanic cavity and external auditory meatus join to form the tympanic membrane.As a result, the tympanic membrane is derived from all three germ layers.