592 



TEXT-BOOK OF EMBRYOLOGY. 



Jacobson's organ develops to a greater degree, and some of the epithelial cells 

 send out processes which pass to the olfactory lobes. 



THE EAR. 

 The ear of higher Vertebrates consists of three parts — the internal, middle, 

 and external. Of these, the internal is the sensory portion proper and, so far 

 as the epithelial elements are concerned, is of ectodermal origin, but secondarily 

 becomes embedded in the subjacent mesoderm. It constitutes a complicated 

 and highly specialized structure for the reception of certain stimuli that are to be 

 conveyed to the central nervous system. From a functional standpoint it may 

 be divided into the portion composed of the semicircular canals and their ap- 

 pendages, which is concerned in receiving and transmitting stimuli destined 



Fig. sir. — Half of a transverse section through the region of the developing ear of a sheep 



embryo of 13 mm. Bbttcher. 



Aud. ves., Auditory vesicle; Co. gang., cochlear ganglion; End. ap., endolymphatic 



appendage; Rh.br., rhombic brain. 



for the static and equilibration centers in the central nervous system, and the 

 cochlear portion, which is concerned in receiving and transmitting auditory 

 stimuli. The middle and outer ear represent modified portions of the most 

 cranial of the branchial arches and grooves, and constitute an apparatus for 

 conducting sound waves to the cochlear portion of the inner ear. 



The Inner Ear.— In embryos of 2 to 4 mm., the ectoderm becomes some- 

 what thickened over a small area lateral to the still open neural groove in the 

 region of the future hind-brain. This thickening is often spoken of as the 

 auditory placode (see p. 506). Owing to more rapid growth of the cells in the 

 deeper layers of the placode, it soon becomes converted into a cup-shaped 

 depression which is known as the auditory pit. The edges of the pit fold 

 in and fuse and the pit thus becomes the auditory vesicle (otocyst), which 

 finally becomes constricted from the parent ectoderm and lies free in the sub- 

 jacent mesoderm (Fig. 511). 



