DEVELOPMENT OF THE SENSE ORGANS 37I 



their peripheral processes either end freely in the epithelium or appropriate new 

 cells to serve as sensory receptors (taste, hearing). 



The nervous structures of the sense organs consist of the general sense organs 

 of the integument, muscles, tendons, and viscera, and of the special sense organs 

 which include the taste buds of the tongue, the olfactory epithelium, the retina, 

 optic nerve and lens of the eye, and the epithehal lining of the ear labyrinth. 



I. General Sensory Organs 



Free nerve terminations form the great majority of all the general sensory 

 organs. When no sensory corpuscle is developed, the neurofibrils of the sensory 

 nerve fibers separate and end among the cells of the epithelia. 



Lamellated corpuscles first arise during the fifth month as masses of meso- 

 dermal cells clustered around a nerve termination. These cells increase in num- 

 ber, flatten out, and give rise to the concentric lamella of these peculiar structures. 

 In the cat these corpuscles increase in number by budding. 



The tactile corpuscles, according to Ranvier, are developed from mesenchymal 

 cells and branching nerve fibrils during the first six months after birth. 



n. Taste Buds 



The anlages of the taste buds appear as thickenings of the lingual epitheUum 

 in 1 10 mm. (C H) fetuses (Graberg) . The cells of the taste bud anlage lengthen 

 and later extend to the surface of the epithehum. They are differentiated into the 

 sensory taste cells, with modified cuticular tips, and into supporting cells. The 

 taste buds are supphed by nerve fibers of the seventh, ninth, and tenth cerebral 

 nerves; the fibers branch and end in contact with the periphery of the taste cells. 



In the fetus of five to seven months taste buds are more widely distributed 

 than in the adult. They are found in the walls of the vallate, fungiform, and 

 foliate papillae of the tongue, on the under surface of the tongue, on both surfaces 

 of the epiglottis, on the palatine tonsils and arches, and on the soft palate. After 

 birth many of the taste buds degenerate, only those on the lateral walls of the 

 vallate and foliate papillae, on a few fungiform papillae, and on the laryngeal sur- 

 face of the epiglottis persisting. 



m. The Olfactory Organ 

 The olfactory epithelium arises as paired thickenings or placodes of the 

 cranial ectoderm (Fig. 369 A). The placodes are depressed to form the olfactory 

 pits, oxfossce, about which the nose develops (Fig. 89). 



