DEVELOPMENT OF THE SENSE ORGANS 387 



epithelium are differentiated: (1) into sense cells with bristle-like hairs at their 

 ends, and (2) into supporting cells. About the bases of the sensory cells nerve 

 fibers from the vestibular division of the acoustic nerve branch. The maculae 

 resemble the cristae in their development save that larger areas of the epithelium 

 are differentiated into cushion-like end organs. Over the maculae concretions of 

 lime salts may form otoconia which remain attached to the sensory bristles. 



The true organ of hearing, the spiral organ, is developed in the basal epithe- 

 lium of the cochlear duct, basal having reference here to the base of the cochlea. 

 The development of the spiral organ has been studied carefully only in the lower 

 mammals. According to Prentiss (Amer. Jour. Anat., vol. 14, 1913) in pig em- 

 bryos of 5 cm. the basal epithehum is thickened, the cells becoming highly colum- 

 nar and the nuclei forming several layers. In later stages, 7 to 9 cm., inner and 

 outer epitheHal 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 swelhngs 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) thick- 

 ening give rise to the epithelium of the spiral limhus, to the cells fining the internal 

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

 386 B, C). The oul^er epitheHal thickening forms the pillars of Corti, the outer 

 hair cells, and supporting cells of the spiral organ. Differentiation 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 

 epitheHal thickening which fie between the labium vestibulare and the spiral 

 organ (Fig. 386 B, C). These ceUs become cuboidal, or flat, and fine the spiral 

 sulcus, while the membrana tectoria loses its attachment with them. The mem- 

 brana tectoria becomes thickest over the spiral organ and in fufi term fetuses is 

 stiU attached to its outer cells (Fig. 386 C). 



Hardesty (Amer. Jour. Anat., vol. 18, 1915), on the contrary, asserts that the membrana 

 tectoria is not attached permanently to the cells of the spinal organ. 



From 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, how- 

 ever, 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 epithefium, 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 



