DEVELOPMENT OF THE SENSE ORGANS 387 



pig embryos of 5 cm. the basal epithelium is thickened, the cells becoming 

 highly columnar and the nuclei forming several layers. In later stages, 

 7 to 9 cm., inner and outer epithelial thickenings are differentiated, the 

 boundary line between them being the future spiral tunnel (Fig. 386 A). 

 At the free ends of the cells of the epithelial swellings there is formed a 

 cuticular structure, the membrana tectoria, which appears first in embryos 

 of 4 to 5 cm. The cells of the inner (axial) thickening give rise to the 

 epithelium of the spiral limbus, to the cells lining the internal spiral sulcus, 

 and to the supporting cells and inner hair cells of the spiral organ (Fig. 

 386 B, C). The outer epithelial thickening forms the pillars of Corti, 

 the outer hair cells, and supporting cells of the spiral organ. Differentia- 

 tion begins in the basal turn of the cochlea and proceeds toward the apex. 

 The internal spiral sulcus is formed by the degeneration and metamorphosis 

 of the cells of the inner epithelial thickening which lie between the labium 

 vestibulare and the spiral organ (Fig. 386 B, C). These cells become 

 cuboidal, or flat, and line the spiral sulcus, while the membrana tectoria 

 loses its attachment with them. The membrana tectoria becomes 

 thickest over the spiral organ, and in full term fetuses is still attached to 



its outer cells (Fig. 386' C). 



-.- 



Hardesty (1915), on the contrary, asserts that the membrana tectoria is not attached 

 permanently to the cells of -the spiral organ. 



Prom what is known of the development of the spiral organ in human embryos, it 

 follows the same lines of development as described for the pig. It must develop relatively 

 late, however, for, in the cochlear duct of a newborn child figured by Krause, the spiral 

 sulcus and the spiral tunnel are not yet present. 



The mesenchyme surrounding the labyrinth is differentiated into a 

 fibrous membrane directly surrounding the epithelium, and into the 

 perichondrium of the cartilage which develops about the whole internal 

 ear. Between these two is a more open mucous tissue which largely 

 disappears, leaving the perilymph space. The membranous labyrinth is 

 thus suspended in the fluid of the perilymph space. The body labyrinth 

 is produced by the conversion of the cartilage capsule into bone. In the 

 case of the cochlea, large perilymph spaces form above and below the cochlear 

 duct. The duct becomes triangular in section as its lateral wall remains 

 attached to the bony labyrinth, while its inner angle is adherent to the 

 modiolus. The upper perilymph space is formed first and is the scala 

 vestibuli; the lower space is the scala tympani. The thin wall separating 

 the cavity of the cochlear duct from that of the scala vestibuli is the 

 vestibular membrane (of Reissner). Beneath the basal epithelium of the 

 cochlear duct, a fibrous structure, the basilar membrane, is differentiated 

 by the mesenchyme. The modiolus is not preformed as cartilage, but 



