Johnston, Morphology of the Head. 219 



very great variation in the relation of the neural crest to the 

 brain and to the ectoderm. The suggestion presents itself that 

 there has been a progressive separation of the material for the 

 ganglia from the brain and, if the acustico-lateral ganglia be in- 

 cluded, it appears that the more highly specialized components 

 came off first, the less specialized last. 



The neural crest is to be regarded as fundamentally a part 

 of the central nervous system. This is evidenced by the pres- 

 ence of its equivalent within the nerve cord in Amphioxus, by 

 the fact that it is more or less completely included in the neural 

 tube in craniata when the tube first rolls up, and by the fact 

 that a part of its cell's remain within the spinal cord in fishes 

 (38, 39, 66). If this view of the neural crest be accepted, is 

 the neuromast system to be considered as a new structure of 

 peripheral origin, or is it to be brought into relation with other 

 structures represented in the neural crest? The view that it is 

 of peripheral origin gives no explanation of its very intimate 

 connections with the centers of the general cutaneous system in 

 the medulla oblongata and cerebellum. The neuromast system 

 must be supposed to have come into relation with the general 

 cutaneous centers by accident and then to have largely usurped 

 them. Even the suggestion made by Koltzoff {^6, p. 502) 

 that the general cutaneous ganglia represent an earlier category 

 of primary sense organs which have been transformed into 

 ganglia does not help in this matter. Yet the relation of the 

 neuromast and general cutaneous components in their nerve 

 centers is of so fundamental a character that the writer is con- 

 vinced that it must be reckoned with as one of the most im- 

 portant factors in the interpretation of the acustico-lateral sys- 

 tem. 



Two structures, both of which are primitive and transient, 

 and both of which point to the ancestral structure of the nerv- 

 ous system, are significant for the explanation of the origin and 

 relationships of the acustico-lateral system: first, the neural 

 plate stage in the ontogeny of the central nervous system, and 

 second, the existence in the spinal cord of fishes of cells whose 

 dendrites are distributed to the skin and whose neurites behave 



