Fig. 300. Section through the head of a Lepidosteus embryo on the sixth day after impregnation.
au.v. auditory vesicle; au.n. auditory nerve; ch. notochord; hy. hypoblast.
Fig. 301. Section through the hind-brain of a Chick at the end of the third day of incubation.
IV. fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle. Ch. notochord; CV. anterior cardinal vein; CC. involuted auditory vesicle (CC points to the end which will form the cochlear canal); RL. recessus labyrinthi (remains of passage connecting the vesicle with the exterior); hy. hypoblast lining the alimentary canal; AO., AO.A. aorta, and aortic arch.
In all Vertebrata the auditory vesicle undergoes further changes of a complicated kind. In the Cyclostomata these changes are less complicated than in other forms, though whether this is due to degeneration, or to the retention of a primitive state of the auditory organ, is not known. In the Lamprey the auditory vesicle is formed in the usual way by an invagination of the epiblast, which soon becomes vesicular, and for a considerable period retains a simple character. As pointed out by Max Schultze, a number of otoliths appears in the vesicle during larval life, and, although such otoliths are stated by J. Müller to be absent both in the full-grown Ammocœte 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 triangle 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.