ORGANS OF SPECIAL SENSE 289 
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 otoeyst 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- 
euished: (a) ductus endolymphaticus 
or recessus labyrinthi; (b) pars su- 
perior labyrinthi; (¢) 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 F ee ae Ee a of te oe 
= é . of a chick embryo short e= 
Endolymphaticus. It was noted in : So eieraae 
z fore its separation from the 
} 
Chapter VI that the ductusendolym- gctoderm. (After Krause.) 
phaticus is united to the epidermis D. e., Endolymphatic duct. 
Ect., Ectoderm. p. v., Pocket 
for formation of vertical semicir- 
lumen up to the stage of 104 hours eular canals. X indicates the 
at least (Fie. 98). Shortly after. this Strand of cells uniting the endo- 
Cat enone , lymphatic duct to the ectoderm. 
by a strand of cells that preserves a 
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 saceus continue 
during the seventh day, and on the eighth day the saccus overtops 
