AUDITORY ORGANS. 519 



of the epiblast, which soon becomes vesicular, and for a consider- 

 able period retains a simple character. As pointed out by Max 

 Schultze, a number of otoliths appears in the vesicle Tluring 

 larval life, and, although such otoliths are stated by J. Muller to 

 be absent both in the full-grown Ammoccete and in the adult, 

 they have since been found by Ketel (No. 387). The formation 

 of the two semicircular canals has not been investigated. 



In all the higher Vertebrates the changes of the auditory 

 sacks are more complicated. The ventral end of the sack is 

 produced into a short process (fig. 301, CC); while at the dorsal 

 end there is the canal-like prolongation of the lumen of the sack 

 (RL\ derived from the duct which primitively opened to the 

 exterior, and which in most cases persists as a blind diverticulum 

 of the auditory sack, known as the recessus labyrinthi or 

 aqueductus vestibuli. The parts thus indicated give rise to 

 the whole of the membranous labyrinth of the ear. The main 

 body of the vesicle becomes the utriculus and semicircular canals, 

 while the ventral process forms the sacculus hemisphericus and 

 cochlear canal. 



The growth of these parts has been most fully studied in 

 Mammalia, where they reach their greatest complexity, and it 

 will be convenient to describe their development in this group, 

 pointing out how they present, during some of the stages in their 

 growth, a form permanently retained in lower types. 



The auditory vesicle in Mammalia is at first nearly spherical, 

 and is imbedded in the mesoblast at the side of the hind-brain. 

 It soon becomes triangular in section, with the apex of the tri- 

 angle pointing inwards and downwards. This apex gradually 

 elongates to form the rudiment of the cochlear canal and sacculus 

 hemisphericus (fig. 302, CC). At the same time the recessus 

 labyrinthi (R.L) becomes distinctly marked, and the outer wall 

 of the main body of the vesicle grows out into two protuberances, 

 which form the rudiments of the vertical semicircular canals 

 ( V.B). In the lower forms (fig. 305) the cochlear process of the 

 vestibule hardly reaches a higher stage of development than that 

 found at this stage in Mammalia. 



The parts of the auditory labyrinth thus established soon 

 increase in distinctness (fig. 303) ; the cochlear canal (CC) 

 becomes longer and curved ; its inner and concave surface being 



