334 



DR. A. MILNES MAKSHALL. 



Head Segments of Vertebrates. 



This table, which has been modelled after the one given 

 by Balfour,^ differs from this latter in some important points. 

 The differences are most marked in the anterior part of the 

 headj where I have added an olfactory segment, and have 

 attempted to define more accurately the constituents of the 

 second segment by removing the fourth and sixth nerves, 

 and assigning, for reasons discussed elsewhere, the third 

 nerve as the true segmental nerve. At the hinder end of the 

 head I have added two segments for the two hinder branchial 

 clefts of Notidanus and the Marsipobranchii, so that the table 

 is intended to include the full number of head segments of 

 which we have any definite indication in any vertebrate, 

 excepting Amphioxus ; though it is by no means intended to 

 exclude the possibility of additional segments having existed 

 at the hinder end of the head in former vertebrates, or 

 actually existing in some living forms. 



It will further be noticed that I have placed the visceral 

 clefts, and not the visceral arches, as indicating segments ; 

 this is a point of some importance, but one which I do not 

 think we are yet in a position to decide with certainty. The 

 question is, whether the visceral clefts are to be viewed as 

 intersegmental, i. e. as corresponding in position to the lines 

 of separation of the original segments by the fusion of which 

 we suppose the vertebrate head to have been formed; or 

 whether they should be considered as inirasegtnental, as 

 apertures formed in the substance of the segments ; in other 



' Op. cit., p. 216. 



