SENSORY ELEMENTS OF THE NOSE, EAR, AND EYE 167 



Sensory elements of the nose / 

 ear, and eye 



The nasal apparatus begins development as a thickening of the ectoderm. 

 This thickening, the nasal placode, is induced by adjacent structures, since 

 transplantations of the head organizer show that nasal placodes may develop 

 from ectoderm of the body region. The nasal placode invaginates to form 

 the nasal pit (Fig. 98), and the cells lining this pit contain the neuroblasts 

 which send fibers into the telencephalon to form the first cranial nerve, the 

 olfactory nerve. This nerve stimulates the further differentiation of the region 

 of the telencephalon into which it enters. 



The origin of the ear is similar to that of the nasal organ. Opposite the 

 hindbrain the ectoderm forms two pits which enlarge and separate from the 

 epidermis as two vesicles — the otic vesicles or otocysts (Fig. 99). These 

 structures are stimulated to form by inductive influences arising both from 

 the hindbrain and the surrounding mesoderm. The otic vesicles become pear 

 shaped as a result of the outgrowth of the endolymphatic duct. From part of 

 the walls of the ear vesicle the three semicircular canals develop, constituting 

 the organ of balance. The ventral wall of the otocyst differentiates into the 

 ductus cochlearis, in which the sensory elements of the ear are formed. The 

 nerves of the ear arise from a ganglion which sends nerves to both the semi- 

 circular canals and the cochlear duct during their differentiation. Fibers also 

 grow back to the hindbrain to form the eighth cranial nerve. 



The eyes appear very early as outgrowths of the forebrain called optic 

 vesicles (Fig. 100). They grow out laterally until they touch the head 

 ectoderm. Where the optic vesicles come in contact with the head ectoderm 

 they stimulate lens formation. The vesicles remain attached to the forebrain 

 by means of the optic stalks. The most distal face of the optic vesicle becomes 

 indented and thus a cup-shaped structure with double walls — the optic cup — 

 is formed. During this process the ventral region near the optic stalk fails to 

 complete the rim of the cup, and as a result a slitlike choroid fissure is left. 

 This choroid fissure serves as an outlet for the optic nerve, and through it 

 mesenchyme and blood vessels enter the optic cup. The lens fits into the cup 

 itself and the overlying ectoderm with some mesenchyme becomes converted 

 into the transparent cornea. 



