Johnston, Morphology of the Head. 223 



the writer and many others have reached in recent years that 

 the acustico-lateral system should be referred strictly to the 

 same segment with the faciaHs. But the reasoning here em- 

 ployed is in perfect harmony with the spirit and method which 

 have led us to that conclusion, and the present result is based 

 on new facts. It should be said that the writer does not accord 

 to the neuromast system any value to segmental study except 

 as a landmark, and that the truth or error of the segmental 

 arrangement suggested here does not in any way affect the 

 validity or the general interpretation of the acustico-lateral sys- 

 tem given above. 



By the relation of this system to the general cutaneous 

 nerves I mean the question whether it corresponds to one or 

 more dorsal rami, or whether it has used up a larger part of the gen- 

 eral cutaneous component. From what goes before it is clear that 

 the neuromast system must be thought to have absorbed a vari- 

 able part of the general cutaneous component in the segments 

 to which the profundus, trigeminus, and facialis belong. It may 

 even be that the whole of the general cutaneous component in 

 the facialis segment has been absorbed, and that this accounts 

 for its apparent total absence in that segment. It is of course 

 evident that the communis component in the neural crest could 

 not enter into the neuromast system, and so all schemes of seg- 

 mentation which raise the VIII or any or all of the lateral line 

 nerves to the dignity of complete segmental sensory roots are 

 faulty. 



//. Segments S and folloiving. 



The processes which have led to the conditions in the caudal 

 part of the head are discussed in the following section. It will 

 be unnecessary to describe in detail each segment and the 

 changes which take place in each. Figure 10 and Table B 

 show the component parts of these segments in lower vertebrates 

 more clearly than a description in the text could make it. For 

 the facts in the higher vertebrates I can do no better than refer 

 the reader to the extensive descriptions and tabular statements 

 of FuRBRiNGER. These must be read, however, it seems to me, 



