466 



HISTOLOGY 



utriculus. Since at one of their ends the superior and posterior ducts unite 

 in a single stalk before entering the utriculus, there are but five openings 

 for the three ducts (Fig 472, D). Near one end of each duct there is a 

 dilatation or ampulla, where nerves terminate. 



/m.t. 



FIG. 471. SECTIONS OF RABBIT EMBRYOS TO SHOW THE DEVELOPMENT OF THE EAR. X 9. 

 A, 9 days, 3.8 mm.; B, 10 days, 3.4 mm.; C, I2j days, 7.5 mm.; D, 14 days, 10 mm. a., Ectoderma 

 epithelium which forms the membranous internal ear; a. bas., basilar artery; ch. t., chorda tympanj. 

 d. c., cochlear duct; d. e., endolymphatic duct; d. s. 1., lateral semicircular duct; d. s. s., superior semi; 

 circular duct; ep., epidermis, fa., facial nerve; meten., metencephalon; m. t. medullary- tube; ph- 

 pharynx. 



d.S.p. 



FIG. 472. LATERAL OR EXTERNAL SURFACES OF MODELS OF THE MEMBRANOUS PORTION OF THE LEFT 



INTERNAL EAR FROM HUMAN EMBRYOS. Different enlargements. (After His, Jr.) 

 A, from an embryo of 6.9 mm.; B, 10.2 mm.; C, 13.5 mm.; and D, 22 mm. am., ampulla; c T., caecum 



vestibulare of d. c., cochlear duct; d. e., endolymphatic duct; d. s. 1., d. s. p., and d. s. s. lateral, 



posterior, and superior semicircular ducts; sac., sacculus; ut., utriculus. 



While the formation of the semicircular ducts is occurring in the upper 

 part of the auditory vesicle, the lower portion elongates and its end be- 

 comes coiled, eventually making two and a half revolutions. The coiled 



