586 TEXT-BOOK OF EMBRYOLOGY. 



The utricle and saccule represent divisions of the portion of the vestibular sac 

 which is known as the atrium, and into which the endolymphatic appendage 

 and cochlea open (see p. 583) . In embryos of about 20 mm. a horizontal con- 

 striction begins to divide the atrium into an upper utricular portion, into which 

 the semicircular canals open, and a lower saccular portion (Fig. 512, /, m). 

 The constriction begins on the side opposite the endolymphatic appendage and 

 gradually extends across the atrium until it finally divides the opening of the 

 endolymphatic appendage into two parts (Fig. 513, a, b, c). One of these 

 parts opens into the utricle, the other into the saccule, the two parts together 

 constituting the utriculo saccular duct. 



As stated before-,- the two- or three-layered epithelium of the earlier stages 

 becomes reduced to a single layer. The cells of this layer are low cuboidal, 

 with the exception of those over small areas in the ampullae, in the saccule, and 

 in the utricle. Over an elongated area in each ampulla (crista ampullaris), a 

 round area in the saccule and another in the utricle (macula acustica), the 

 epithelium becomes high columnar, some of the cells developing cilia on their 

 free borders ("hair cells," neuro-epithelium) , the others becoming the susten- 

 tacular cells. These areas are the end-organs of the vestibular nerve (see p. 499) . 



As already mentioned, the cochlear pouch appears as an outgrowth from the 

 lower side of the atrium (see also Fig. 512, b-f) . The pouch becomes somewhat 

 flattened, and, as it continues to grow in length, becomes coiled like a snail- 

 shell (Fig. 512, g-n; Fig. 513, a-c). This first formed coiled structure is the 

 cochlear duct, or scala media. At the same time, it becomes distinctly marked 

 off from the lower part of the atrium (now the saccule) by a constriction, the 

 constricted portion forming the ductus reuniens (Fig. 512, l-n; Fig 513, a-c). 



All the structures thus far considered are at first closely invested by meso- 

 derm. Later, this portion of the mesoderm gives rise to special tissues, and, in 

 the region of the cochlear duct, to the scala vestibuli and scala tympani. The 

 cells immediately around the vesicle proliferate and a dense fibrous layer is 

 formed; outside of this fibrous layer the tissue becomes gelatinous; outside of 

 this again another fibrous layer is formed, around which cartilage develops. 

 The inner fibrous layer gives rise to the connective tissue that supports the 

 epithelial lining of the vesicle. The gelatinous layer degenerates to form a 

 fluid known as the peritymfb, the space containing the fluid being the perilymph- 

 atic space. The outer fibrous layer becomes the perichondrium later the 

 periosteum when the cartilage is replaced by the petrous portion of the tem- 

 poral bone. 



In the cochlear region the conditions are somewhat modified. Here the 

 gelatinous layer does not form a complete covering for the cochlear duct, but is 

 interrupted along two lines, (i) Laterally the fibrous layer lying next the 

 cochlear duct is fused with the perichondrium (outer fibrous layer) (Fig. 514). 



