220 Journal of Comparative Neurology and Psychology. 



as do those of the general cutaneous ganglion cells. The first 

 indicates that the nervous system was in primitive vertebrates a 

 specialized area of the superficial ectoderm. The second fact 

 indicates that the primitive form of cutaneous innervation was 

 by means of dendrites of cells in this specialized area growing 

 out to the rest of the ectoderm. This general cutaneous innerv- 

 ation corresponds to the free nerve endings in invertebrates 

 which come from cells in the central nervous system. That this 

 is the primitive form of innervation for tactile and pressure 

 stimuli in vertebrates, I have no doubt. The peripherally situ- 

 ated tactile cells (hair cells) of the highly specialized arthropods 

 do not appear to me to be in the line of vertebrate development. 

 Granted that the sensory cells which supply free nerve endings 

 to the ectoderm in worms may (or must) once have been derived 

 from the ectoderm, we must nevertheless believe that since these 

 cells gained their position in the central nervous system in the 

 worms that relation has been maintained in the line of verte- 

 brate development. Now the relation of the neuromast system 

 in present vertebrates to the general cutaneous system compels 

 us to seek the origin of the neuromast system as a modification 

 of that already established cutaneous system. This seems the 

 more necessary on account of the phylogenetically late appear- 

 ance of the neuromast system. 



If the line of reasoning thus far is good, we have to make 

 only two suppositions for the explanation of the chief relation- 

 ships of the acustico-lateral system. First, the area from which 

 the system has been developed originally belonged to the neural 

 plate area ; Second, there have appeared in this area a special 

 type of sense cells characteristic of the neuromast organs. It 

 is not hard to conceive that the neuromast area once formed the 

 marginal portion of the specialized ectoderm which constituted 

 the central nervous system. The idea that the neural crest and 

 neuromast area together formed a marginal band along the 

 primitive neural plate is supported by the position of the neuro- 

 mast area just behind the broad cephalic plate and by the fact 

 that opposite this area there is a long space in which the neural 

 crest contains no cutaneous elements and in which it is partially 



