The ear is developed chiefly from the epiblast, a special and independent involution of which forms both its essential nervous structures and the general epithelium lining the membranous labyrinth. The mesoblast supplies the surrounding firmer structures, such as the fibrous substance of the inner ear, and the bony parts in which the organ lies. The auditory nerve grows as a.bud from the neural tissue forming the hind-brain, and connects it with the delicate specialized auditory cells.
Fig. 314. Transverse section through the head of a foetal sheep in the region of the hind-brain.
H.B. Hind-brain. CC. Canal of the cochlea. RV. Recessus vestibuli. VB. Vertical semicircular canal. G.C. Auditory ganglion. G'. Auditory nerve. N. Connection of auditory nerve to the hind-brain.
The process begins by the appearance of a depression of the general epiblast covering the head, which forms a tubular diverticulum, lying in the mesoblast at the side of the hind-brain.
This diverticulum becomes separated from the epiblast by the obliteration of its outer extremity, which united it to the superficial epiblast, and is converted into a cavity receiving the name of the otic vesicle. It soon becomes somewhat triangular in shape, the base of the triangle lying upward.
From the lower angle arises a projection, which is the rudimentary canal of the cochlea. The angle lying next to the neural epiblast similarly gives off a tubular process, which forms the recessus vestibuli.
Elevations in the primitive vesicle indicate the origin of the semicircular canals, which become tubular, opening at their ends into the general cavity of the vesicle. The two superior canals are the first to appear, the horizontal arising somewhat later.
The part of the otic vesicle in connection with the canal of the cochlea becomes separated from the latter by a narrow constriction which forms the canalis reuniens, the part of the vesicle beyond this developing into the saccule.
The utricle arises from that part of the vesicle which is in connection with the semicircular canals. It is at first in direct connection with the saccule, but after a time it only communicates by means of a narrow canal with a similar one from the saccule; these two canals are connected with a third, which lies in the aqueductus vestibuli.
The canal of the cochlea is at first a straight tube, but as it develops it becomes coiled upon itself.
The walls of the primitive otic vesicle, formed from the epiblast, become developed into the epithelium lining the internal ear. The mesoblast immediately surrounding the vesicle forms a supporting capsule of fibrous tissue, which completes the membranous parts of the internal ear.
Part of the mesoblast around the optic vesicle becomes liquefied, and gives origin to the canals and spaces in which the membranous labyrinth lies; the neighboring mesoblast is changed into cartilage which ossifies and forms the bony parts of the ear.
The auditory nerve is developed from the hind-brain, and grows through the mesoblast toward the otic vesicle. It is recognizable from its having some ganglion cells in its growing extremity from a very early period of its development.
The Eustachian tube and the tympanum are formed in connection with the inner part of the first visceral cleft, and the ossicles are developed from the corresponding visceral arch - hyo-mandibular.
Fig. 315. Section through the head of a foetal sheep. (Boettcher.) R. V. Recessus vestibuli. V. B. Vertical semicircular canal.- H. B. Horizontal semicircular canal. G. Auditory ganglion. C.C. Canal of the cochlea.
The membrana tympani is formed at the surface of the embryo, the adjacent parts grow outward and give rise to the external auditory meatus.