The development of the respiratory system begins during the fourth week of development as an evagination of the

The respiratory diverticulum is an evagination of the endodermal floor of the foregut. All of the epithelial surfaces of the respiratory system are derived from this evagination. The first branchial pouch is the endodermal imagination that forms the eustachian tube and the middle ear linings. The first branchial cleft is an ectodermal imagination that forms the external ear canal. The sixth branchial arch contributes to the cartilage of the larynx.