600 TEXT-BOOK OF EMBRYOLOGY. 



that the base of the stapes is of intramembranous origin and that the rest of the 

 bone is derived from the cartilage of the second arch. Its close association 

 with the cartilage of the second arch possibly indicates its phylogenetic origin 

 from the latter. 



At first the auditory ossicles are embedded in the mesoderm dorsal to the 

 first inner branchial groove, that is, dorsal to the cavity of the middle ear. As 

 development proceeds, the mesoderm is converted into a spongy tissue which 

 finally degenerates. At the same time the ear cavity enlarges and wraps itself, 

 as it were, around the ossicles. The latter thus come to lie within the cavity 

 of the tympanum, but are covered by a layer of epithelium (entoderm) which 

 is continuous with that lining the cavity. 



Toward the end of fcetal life, outgrowths from the cavity of the tympanum 

 begin to invade the temporal bone. This process continues for some time 

 after birth and results in the formation of cavities within the mastoid part of 

 the temporal bone. These cavities are the mastoid cells, the epithelial lining 

 of which is continuous with that of the tympanic cavity. 



The Eustachian tube represents the lower (ventral) portion of the diver- 

 ticulum which forms the cavity of the tympanum. In other words, as the 

 dorsal part of the first inner branchial groove enlarges to form the cavity of the 

 middle ear, the narrow part of the groove, just ventral to the cavity, persists 

 as a communication between the latter and the pharynx. 



The Outer Ear. The outer ear is formed from the dorsal part of the first 

 outer branchial groove and the adjacent portions of the first and second arches 

 (see Fig. 123). The ventral part of the groove flattens out and disappears. 

 The dorsal part becomes deeper to form a funnel-shaped depression (during 

 the second month; Fig. 126). From the deeper part of the funnel a solid mass 

 of ectoderm grows inward until it comes into relation with the mesoderm im- 

 mediately around the developing cavity of the tympanum, or, more specifically, 

 the mesoderm surrounding the handle of the malleus. Here it spreads out 

 into a disk-like mass. About the seventh month, the disk splits into two layers. 

 The inner layer, which is separated from the epithelium of the middle ear by a 

 thin sheet of mesoderm, becomes the outer layer of the tympanum. The 

 tympanum is thus composed of an inner (entodermal) and an outer (ectoder- 

 mal) layer, with a small amount of mesoderm between. From its mode of 

 development, the tympanum may be considered in a sense as the wall which 

 separates the first inner from the first outer branchial groove. 



The split in the ectodermal disk (see above) gradually extends outward, 

 invading the solid ectodermal invagination until it finally unites with the 

 bottom of the funnel-shaped depression on the surface, thus forming the 

 external auditory meatus. 



The external ear (or auricle) is derived from the portions of the first and 



