SENSE OR RECEPTOR ORGANS 843 



lying deep within the dermis (fig. 359F). The pit organs, ampullae, etc., lo- 

 cated over the head region of fishes belong to the lateral-line system. They 

 are highly specialized structures. A developmental stage of the lateral-line canal 

 in Squalus acanthias are shown in figure 359G. 



5. The Taste-bud System 



The taste-bud system of vertebrates is most variable in its distribution. In 

 mammals the taste buds are scattered over the tongue (fig. 285E), and upon 

 the larynx, pharynx and soft palate. The taste buds on the anterior portion 

 of the tongue are supplied by the chorda tympani branch of the facial nerve, 

 the posterior lingual taste buds by the glossopharyngeal, and those in the 

 region of the pharynx by the vagus. In most fishes the taste buds are spread 

 over the inner surfaces of the pharynx and extensively over the buccal cavity. 

 In some fishes and amphibia they appear also over the external surface of 

 the head, and in some teleosts they are found over much of the body surface 

 (see figure 356C). 



Taste buds consist of groups of specialized columnar epithelial cells, known 

 as hair cells, surrounded and supported by sustentacular cells. Each hair cell 

 has a sensory bristle protruding to the surface, whereas basally it is in contact 

 with dendritic terminalizations of sensory nerves (fig. 359H). 



6. The Development of the Olfactory Organ 



The senses of smell and taste are much alike. Both detect chemical sub- 

 stances dissolved in fluid. The olfactory epithelium of the vertebrate group 

 is of the simple columnar variety containing neurosensory cells (fig. 356A) 

 supported by non-nervous epithelial elements. Each neurosensory cell at its 

 free surface terminates in a series of cilia-like structures, and at its basal end 

 is prolonged into a neurite (nerve fiber) which passes into the olfactory bulb 

 where it breaks up into a number of telodendria. The olfactory area of the 

 human nasal passageway is shown in figure 3591 (see legend). 



a. Development of the Olfactory Organs in Squalus acanthias 



The two olfactory sacs in Squalus develop as invaginations of a thickened 

 olfactory placode on either side of the antero-ventral aspect of the head near 

 the oral invagination. They remain as blind sacs, extensively folded internally 

 and closely associated with the olfactory bulbs of the brain (fig. 357H). 



b. Development of the Olfactory Organs in the Frog 



The olfactory organs in the frog arise from two placodes, one on either 

 side of the head immediately in front of the developing eyes. These placodes 

 invaginate, and push downward and posteriad toward the developing oral 

 cavity. At about the 10 to 12 mm. stage they perforate into the anterior end 

 of the oral cavity. The walls of the olfactory inpushing become folded to form 



