DEVELOPMENT OF THE EAR. 



379 



of each, an ascending tubular outgrowth, the endolymphatic duct, arises, 

 and its blind termination becomes enlarged to form the endolymphatic 

 sac. The duct is seen in section in Fig. 429, C, and its upper end projects 

 above the rest of the vesicle in Fig. 430, A. In the adult it terminates 

 just beneath the dura. 



In two places the medial and the lateral walls of the upper half of the 

 vesicle approach one another, and after fusing they become thin and rup- 

 ture so that two semicircular ducts are formed (Fig. 430, B and C). The 

 space encircled by each duct may be regarded as a hole in the vesicle. 



-d.e. 



d.s.s. 



FIG. 429. SECTIONS OF RABBIT EMBRYOS TO SHOW THE DEVELOPMENT OP THE EAR. X9. 



A, 9 days, 3.8 mm.; B, 10 days, 3.4 mm.; C, 12^ days, 7.5 mm.; D, 14 days, 10 mm. a., Ectodermal 

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

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

 semicircular dct; ep., epidermis; fa., facial nerve; meten., metencephalon ; m. t., medullary tube; 

 ph., pharynx. 



The two ducts in question are the superior and posterior semicircular ducts 

 respectively. The third or lateral semicircular duct forms soon afterwards. 

 In Figs. 429 D and 430 B it is a horizontal shelf-like projection of the 

 vesicle, the center of which is to become perforated so that its rim forms the 

 duct. The portion of the vesicle which receives the terminal openings 

 of the three semicircular ducts is called the 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. 

 430 D). Near one end of each duct there is a dilatation or ampulla, 

 where nerves terminate. 



