194 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



in which is seated a lenticular tactile cell (fig. 199, A). Somewhat 

 allied are Grandry's (Merkel's) corpuscles in which two or more 

 tactile cells are enclosed in a connective- tissue sheath, while the nerve, 

 losing its medullary sheath as it reaches the capsule, expands to 

 plates which are inserted between each two tactile (fig. 199, B) cells. 

 In another series of sensory structures the end of the nerve is club- 

 shaped and is surrounded by a connective-tissue sheath, either simple 

 (cylindrical corpuscles), or in Pacini's (Vater's, fig. 200) and 

 Herbst's corpuscles, the sheath is formed by layers of cells, recalling 

 the coats of an onion, while immediately around the club is a layer 

 of cubical cells. Still another variant is found in Krausse's (cor- 

 pusculum bulboidemn) and Meissner's corpuscles, where the nerve, 



Fig. 199. — A, tactile corpuscle; 

 B, Grandry's corpuscle. 



Fig. 200. — Vater-Pacinian corpuscle. 



on entering the corpuscle, breaks up into numerous branches which 

 surround an axial core of large cells. 



It is impossible at present to state with certainty the function of 

 each of these and other nerve-end apparatuses and to say which are 

 connected with the different senses — tactile, pressure, pain, heat and 

 cold, muscular, etc. — which are commonly confused under the term 

 'touch.' 



LATERAL-LINE ORGANS 



The lateral-line organs occur only in the ichthyopsida and here 

 only during the branchiate stages. They arise as thickenings of the 

 ectoderm on either side of the head in the neighborhood of the ear. 

 From here the thickenings extend in definite lines which determine 

 the series of organs in the adult. At points on these lines the sensory 

 areas are developed by the differentiation of two kind of cells, the 

 supporting cells, which extend through the epidermis from the corium 

 to the free surface, and the sensory cells, which reach from the surface 

 only part way to the base. The latter are pear-shaped and bear 



