DEVELOPMENT OF THE SENSE ORGANS 371 



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 of the eye, and the epithelial 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 

 mesodermal cells clustered around a nerve termination. These cells in- 

 crease in number, flatten out, and give rise to the concentric lamellae 

 of these peculiar structures. In the cat these corpuscles increase in num- 

 ber by budding. 



The tactile corpuscles, according to Ranvier, are developed from mes- 

 enchymal cells and branching nerve fibrils during the first six months 

 after birth. 



II. TASTE BUDS 



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

 epithelium in three month fetuses. The cells of the taste bud anlage 

 lengthen and later extend to the surface of the epithelium. They are 

 differentiated into the sensory taste cells, with modified cuticular tips, and 

 into supporting cells. The taste buds are supplied 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 dis- 

 tributed 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 surface of the epiglottis persisting. 



III. 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, or fosses, about which the nose develops (Fig. 89). 



In embryos of 4 to 5 mm. (Fig. 369) the placodes are sharply marked 

 off from the surrounding ectoderm as ventro-lateral thickenings near the 

 top of the head. They are flattened and begin to invaginate in embryos 



