. The development of the chick; an introduction to embryology . iate form, so that it consists of a centi-al ca\ityand thi-ee simises, \-iz., the median siiuis which i-ec(>ives the endof the anterior and ])ostei-ior semicircular canals, the jjosteriorsimis situated abo\( tlu> ampulla of tlu; external semicircularcanal, and the antei-iof sinus in the region of the ampulhc of thehorizontal and sagit tal semicii-culai- canals (cf. Fig. 17o). A shortdistance in front of the j)osterior siiuis on the median face of ORGANS OF SPECIAL SENSE 293 the utricuhis occur the openings of the ductus en(l
. The development of the chick; an introduction to embryology . iate form, so that it consists of a centi-al ca\ityand thi-ee simises, \-iz., the median siiuis which i-ec(>ives the endof the anterior and ])ostei-ior semicircular canals, the jjosteriorsimis situated abo\( tlu> ampulla of tlu; external semicircularcanal, and the antei-iof sinus in the region of the ampulhc of thehorizontal and sagit tal semicii-culai- canals (cf. Fig. 17o). A shortdistance in front of the j)osterior siiuis on the median face of ORGANS OF SPECIAL SENSE 293 the utricuhis occur the openings of the ductus en(lolyni]),saccuhis, and ductus cochlearis; the two hitter derived from thepars inferior of tiie , to tlie devclopiuent of which wenow turn. (c) Development of the Pars Inferior Lahi/rinthi; Lagena,Ductus Cochlearis, and Sacculus. During the changes descri])edin the pars superior hibyrinthi, the pars inferior has developedinto the ductus cochlearis and lagena on the one hand, and thesacculus on the other. Throughout the series of the vertebrates. Fig. 171. — Model of tlie auditory lal)yrinth of the left side of a chickof 7 days and 17 hours. A. M(>dian view. B. External view.(After Rothig and Brugsch.)A. a., Ampulla of the anterior semicircular canal. A. p., Ampulla of the posterior semicircular canal. C. a., Anterior semicircular canal. C. 1., Pocket for formation of the lateral semicircular canal. C. p., Pocket for formation of the posterior semicircular canal. Sa., Sacculus. Other abbreviations as Ixfore. the structure of the pars supeiior is very uniform; the pars inferior,on the other hand, has a characteristic structure in each classand exhibits in general a progressive evolution. The conditionin the chick is characteristic on the whole for the class of six days the lower division of the otocyst has gi-own outventralward into a deep pouch which is curved jjostcriorly andtowards the middle line (Fig. 170); the terminal })()rtion is therudiment o
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