The development of the organ of hearing is somewhat complicated. The first part to appear is a portion of the inner ear, which shows itself as a round, hollowing of the ectoderm. This depression becomes deeper and sinks further in, while its floor becomes thicker, and finally the whole assumes the shape of a closed cavity. An outgrowth from this gives rise to the _cochlea_. The cavity becomes divided into two portions, in one of which the _semicircular canals_ arise. Around the whole, the embryonic tissue has been forming into a strong protective covering, some of which finally becomes cartilage, and some bone. The middle portion of the ear is the remains of a cleft in the side of the embryo. This cleft becomes changed into a canal by the closing of its edges, the upper part ultimately forming the _tympanic cavity_, and the rest of it remaining as the _Eustachian canal_. This canal opens into the pharynx. In the cavity there are subsequently developed three small bones which play an important part in the process of hearing. After the birth of the embryo, air reaches the tympanic cavity, which then enlarges. One of the walls of this cavity persists as the tympanic membrane or drum. Finally the outer ear, that portion which is popularly spoken of as _the_ ear, is formed from the upper portion of the same cleft which gave rise to the tube of the tympanum. We have referred in the preceding description to the origin of some embryonic structures from a cleft in the early embryo itself. As a matter of fact, no less than four of these clefts, or fissures, appear in the region of the neck on each side, and are of the very greatest interest and importance in connection with embryology. They are termed the "branchial clefts," and are seen in the embryos of all vertebrates. In the human embryo there are four. They are situated on each side of the pharynx, and they correspond to the gill slits in lower vertebrates.