580 comparative anatomy 



Development of Taste Organs in Man 



The lingual papillae upon which the taste buds develop first appear in 

 a nine weeks embryo as minute elevations on the surface of the tongue. 

 The formation of circumvallate papillae involves not only similar eleva- 

 tions of the surface, but also the ingrowth of the epidermis around each 

 papilla. By the subsequent splitting of this ring of epidermis into an inner 

 and outer layer separated by a circular groove, the papilla becomes vallate. 

 The taste buds are differentiated on the sides of the papillae, which are 

 covered by a stratified epidermis of endodermal origin. Each taste bud 

 appears primarily as a local thickening of the epidermis, due to the elonga- 

 tion of the cells which are to become the sensory and supporting cells. 

 During the fifth to the seventh month of fetal life, taste buds are more 

 numerous than later, and occur very generally over the surface of the 

 mouth and pharynx. Later the number diminishes and each taste bud 

 requires an external pore opening to the surface. 



VISUAL ORGANS 



Sensitivity to light is a wide-spread, if not a universal, property of 

 living cells. An amoeba will not enter strong light; and sea anemones, 

 among other creatures, react to light, although they have no specialized 

 photoreceptors. All flagellates which are phototropic have a red pigment 

 spot or stigma like that of Euglena; and in general, in multicellular crea- 

 tures, only those cells respond to light which contain some such pigment 

 as visual purple, which is altered by light. 



The so-called eye-spots of coelenterates are pigment cells which 

 respond to changes in intensity of light. Many medusae have clusters of 

 pigmented columnar epithelial cells on the margin of the umbrella, which 

 are interpreted as photic organs, since the animal ceases to be phototropic 

 when these are removed. The jelly-fish Nausithoe has a lens associated 

 with each of these pigment spots. 



Many species of flatworms have paired pigment spots closely associated 

 with the brain, which are interpreted as eyes. In some species, beaker- 

 like clusters of pigmented cells surround the terminations of sensory nerves, 

 and may, therefore, be regarded as among the most primitive of true eyes. 



While, however, photoreceptors occur in coelenterates and all phyla 

 above, true vision is limited to those forms in which the photoreceptors are 

 aggregated into eyes capable of forming an image on a retina. Among 

 invertebrates two types of eyes predominate, beaker eyes and vesicular 

 eyes. Beaker eyes get their name from their shape. They usually, have 

 a core or plug of cuticula. Vesicular eyes, however, have a liquid-filled 

 cavity around which the photoreceptor cells are arranged. 



The eyes of annelids are varied. Some free-swimming forms have 

 beaker-eyes with spherical lenses and a layer of retinal cells connected by 



