1032 ORGANS OF THE SENSES AND THE COMMON INTEGUMENT 



effected by a fold which extends deeply into the proximal part of the ductus 

 endolymphaticus, with the result that the utricle and saccule ultimately com- 

 municate with each other by means of a Y-shaped canal. The saccule opens 

 into the cochlear duct, through the canalis reuniens, and the semicircular ducts 

 communicate with the utricle. 



Ductus endolymphaticus 



Superior semi- 

 circular duct 





Ganglion 

 cochleare 



Utricle 

 Saccule 

 Lateral semi- 

 circular duct 



Ductus cochlearis 



FIG. 902. Transverse section through head of fetal sheep, in the region of the labyrinth. X 30. (After Boettcher.) 



The mesodermal tissue surrounding the various parts of the epithelial labyrinth 

 is converted into a cartilaginous ear-capsule, and this is finally ossified to form the 

 bony labyrinth. Between the cartilaginous capsule and the epithelial structures 

 is a stratum of mesodermal tissue which is differentiated into three layers, viz., 



Embryonic 

 connective tissue 



Ligamentum spirale 

 Scala tympani 



Epithelium of the spiral 

 organ of Corti 



FIG. 903. Transverse section of the cochlear duct of a fetal cat. (After Boettcher and Ayres.) 



an outer, forming the periosteal lining of the bony labyrinth; an inner, in direct 

 contact with the epithelial structures; and an intermediate, consisting of gelatinous 

 tissue: by the absorption of this latter tissue the perilymphatic spaces are developed. 

 The modiolus and osseous spiral lamina of the cochlea are not preformed in cartil- 

 age but are ossified directly from connective tissue. 



