The tubotympanic recess widens to form the tympanic cavity. Meanwhile, the ossicles of the middle ear have differentiated from the blastema to become cartilaginous. They become integrated into the tympanic cavity and covered with endodermal epithelium. The stapes is situated on the fenestra vestibuli (foramen ovale, oval window), the beginning of the scala vestibuli of the internal ear. The fenestra cochleae (foramen rotundum, round window, end part of the scala tympani) is closed by the epithelium of the tympanic cavity. Later on, the tympanic cavity widens into the petrous bone and forms the mastoid cells.
The external acoustic meatus deepens through proliferation of its ectoderm. In the region of the tympanic membrane, the ectoderm and endoderm are very close to one another. They are separated by only a thin layer of connective tissue, called the stratum fibrosum.