XII.] THE MEMBRANOUS LABYRINTH. 393 



circular canal. At the same time the central parts of 

 the walls of the flat bulgings of the vertical canals grow 

 together, obliterating this part of the lumen, but leaving 

 a canal round the periphery ; and, on the absorption of 

 their central parts, each of the original simple bulgings 

 of the wall of the vesicle becomes converted into a true 

 semicircular canal, opening at its two extremities into 

 the auditory vesicle. The vertical canals are first es- 

 tablished and then the horizontal canal. 



Shortly after the formation of the rudiment of the 

 horizontal semicircular canal a slight protuberance be- 

 comes apparent on the inner commencement of the 

 cochlear canal. A constriction arises on each side of 

 the protuberance, converting it into a prominent hemi- 

 spherical projection, the sacculus hemisphericus (Fig. 

 131 8E). 



The constrictions are so deep that the sacculus is 

 only connected with the cochlear canal on the one hand, 

 and with the general cavity of the auditory vesicle on 

 the other, by, in each case, a narrow short canal. The 

 former of these canals (Fig. 131 6) is known as the 

 canalis reuniens. 



At this stage we may call the remaining cavity of 

 the original otic vesicle, into which all the above parts 

 open, the utriculus. 



Soon after the formation of the sacculus hemispheri- 

 cus, the cochlear canal and the semicircular canals 

 become invested with cartilage. The recessus labyrinthi 

 remains however still enclosed in undifferentiated meso- 

 blast. 



Between the cartilage and the parts which it sur- 

 rounds there remains a certain amount of indifferent 



