ORGANS OF SPECIAL SENSE 



289 



ECt 



drawn into the service of the otocyst in the course of evolu- 

 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 

 by a strand of cells that preserves a 

 lumen up to the stage of 104 hours 

 at least (Fig. 98). Shortly after, this 

 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 sixth day the 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 the enlargement of the saccus continue 

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



FIG. 167. Model of the otocyst 

 of a chick embryo shortly be- 

 fore its separation from the 

 ectoderm. (After Krause.) 



D. e., Endolymphatic duct. 

 Ect., Ectoderm, p. v., Pocket 

 for formation of vertical semicir- 

 cular* canals. X indicates the 

 strand of cells uniting the endo- 

 lymphatic duct to the ectoderm. 



