470 THE ORIGIN OF VERTEBRATES 



predominate over the muscular or glandular, as in the formation of the 

 electric organs of the Torpedo and Malapterurus, but still the final 

 effect is produced by the alteration of the muscle or gland-cell. On 

 the afferent side especially free nerve-terminations are largely recog- 

 nized, or, as in Barker's book, nerves are spoken of as arising in 

 connective tissue. Thus the numerous kinds of special sense-organs, 

 such as Pacinian bodies, tendon-organs, genital corpuscles, etc., are 

 all referred to by Barker under the heading of " sensory nerve 

 beginnings in mesoblastic tissues." Yet the type of these organs 

 has been known for a long time in the shape of Grandry's corpuscles 

 or the tactile corpuscles in the duck's bill, where it has been proved 

 that the nerve terminates in special large tactile cells derived from 

 the surface-epithelium. 



So also with all the others, further investigation tends to put 

 them all in the same category, all special sensory organs originating 

 from a localized patch of surface- epithelium. Thus Anderson has 

 shown me in his specimens how the young Pacinian body is 

 composed of ro-ws of epithelial cells, into each of which a twig 

 from the nerve passes. He has also shown me how, in the case of 

 the tendon-organ, each nerve-fibre passes towards the attachment of 

 the tendon and then bends back to supply the tendon-organ, thus 

 iudicating, as he suggests, how the nest of epithelial cells has 

 wandered inwards from the surface to form the tendon-organ. Again, 

 Meissner's corpuscles and Herbst's corpuscles are evidently referable 

 to the same class as those of Grandry and Pacini. 



Yet another instance of the same kind is to be found in the 

 chromatophores of the frog and other animals which are under the 

 influence of the central nervous system and yet have been supposed 

 by various observers to be pigmented connective tissue cells. The 

 most recent w r ork of Leo Loeb and others has conclusively shown 

 that such cells are invariably derived from the surface-epithelium. 



Finally, in fishes we find the special sense-organs of the lateral 

 line and other accessory sensory organs, all of which are indisputably 

 formed from modified surface epithelial cells. 



The whole of this evidence seems to me directly against Barker's 

 classification of sensory nerve-beginnings in mesoblastic tissues ; in 

 none of these cases are we really dealing with free nervous tissue 

 alone, the starting point is always a neuro-epithelial couple. 



We may then, I would suggest, look upon the adult as formed of 



