ORGAXS OF SPECIAL SEXSE 



289 



drawn into the service of the 'otocyst in the course of evoKi- 

 tion. 



Development of the Otocyst and Associated Parts. In the pre- 

 ceding chapter we took up the formation of the otocyst and the 

 origin of the endolymphatic duct. The latter is at first an apical 

 outgrowth from the otocyst, but its attachment soon becomes 

 shifted to the median side of the otocyst, owing to the expansion 

 of the dorsal external wall of the 

 latter (Fig. 167). Three divisions 

 of the otocyst may now be distin- 

 guished: (a) ductus endolymphaticus 

 or recessus labyrinthi; (6) pars su- 

 perior labyrinthi; (c) pars inferior 

 labyrinthi. The boundary between 

 the two latter is rather indistinctly 

 indicated at this stage by a shallow 

 groove on the median face of the 

 otocyst. The development of these 

 parts may now be followed separately. 



(a) The Development of the Ductus 

 Endolymphaticus. It was noted in 

 Chapter VI that the ductus endolym- 

 phaticus is united to the epidermis 



Fig. 167. — Model of the otocyst 

 of a chick embryo shortly be- 

 fore its separation from the 

 ectoderm. (After Krause.) 



D. e., Endolymphatic duct, 

 bv a strand of cells that preserves a ?^V Ectoderm, p. v., Pocket 

 , " ^ . 1 , / -. r. . 1 *or formation of vertical semicir- 



lumen up to the stage of 104 hours cular canals. X indicates the 



at least (Fig. 9S). Shortly after, this ft^^nd of cells uniting the endo- 



, . , , , lymphatic duct to the ectoderm, 



connection is entirely lost. 



The opening of the endolymphatic duct into the otocyst 



appears to be shifted more and more ventrally along the median 



surface, with the progress of differentiation of the other parts 



of the otocyst, until it lies in the region of communication of 



the utriculus, sacculus and lagena (Figs. 168 and 171). This is 



brought about by the various foldings and expansions of the 



wall of the otocyst described in b and c. In the meantime the 



endolymphatic duct has increased in length with the growth of 



the surrounding parts, and on the sixtli day tlie distal half begins 



to expand to form the saccus endolymphaticus, lying between 



the utriculus and the hind-brain. The elongation of the entire 



endolymphatic duct and tlie enlargement of the saccus continue 



during the seventh day, and on the eighth day the saccus overtops 



