V.] THE SEMICIRCULAR CANALS. 115 



from which it soon becomes definitely marked off by a constric- 

 tion as the ductus cochlearis (Fig. 35, C. (7.), leaving the rest of 

 the vesicle to form the vestibule and its appendages. About the 

 same time that the ductus cochlearis is developing the upper 

 and outer corner of the triangular vestibule grows out up- 

 wards and backwards, as a long hollow process, the recessus 

 or aqueductus vestibuli (Fig. 35, -RF, 36, RV), marked 

 where it leaves the vestibule by a more* or less prominent 

 constriction. The upper end of this is the remnant of the 

 primitive opening of the auditory involution. 



On the remaining surface of the vestibule two hollow 

 protuberances are visible (Fig. 34, V.B.], the rudiments of 

 the two vertical semicircular canals. Below this, a little later 

 on, a similar bulging starts to form the horizontal semi- 

 circular canal (Fig. 35, H.B.). Each protuberance grows 

 out, becomes flattened, and while remaining attached to the 

 vestibule at its two ends becomes separated from it in the 

 middle, and is thus converted into a single tube opening at 

 both ends (one of which subsequently dilates into an ampulla) 

 into the cavity of the vestibule. 



Near the junction of the ductus cochlearis with the 



vestibule, two constrictions give rise to a prominence lying 



between them, and known as the sacculus hemisphericus. This, 



though very conspicuous in mammals (Fig. 36, S.B.}, is very 



slightly marked in birds (Fig. 33, s). The rest of the vesti- 



i bule remains as the utriculus. The progressive constriction 



I at the base of the ductus cochlearis gives rise to the canalis 



i reuniens. Thus, by an unequal growth resulting in these 



protuberances and constrictions, the originally oval sac is 



I modelled into the membranous labyrinth. 



While the vesicle is thus increasing in size and changing 

 in form, a large quantity of mesoblast is gathered round it. 

 The outer portion of this mesoblastic investment becoming 

 dense is converted into cartilage, while the internal portion 

 remains as undifferentiated tissue. Later on, the fnost in- 

 ternal layers of this undifferentiated tissue are converted 

 into the membranous coat (corium) which supports the 

 epithelium of the membranous labyrinth ; while the re- 

 mainder is liquefied or absorbed at the same time that the 

 cartilaginous envelope becomes ossified, and so gives rise to 

 the cavity of the osseous labyrinth with its contained peri- 



