THE SEGMENTATION OF THE HEAD. 



diagram. Among the interesting features is the recognition 

 of the olfactory as a segmental nerve ; though the nasal 

 organ is not regarded as a branchial cleft, but as a sense- 

 organ (cf. Marshall). The facial is also regarded as a compound 

 nerve, and the auditory nerve is segmental because it supplies 

 the ear, which, like the nose, is in the same category of seg- 

 mental sense-organs. 



In the year preceding Beard's paper Ahlborn maintained that 

 there were two distinct kinds of segmentation in the head, the 

 one of the mesoderm, mesomery, the other of the alimentary 

 canal, branchiomery, and that these two were independent and 

 not causally related. His views have had not a little influence 

 on subsequent work, but it must be said that it is not a difficult 

 matter to answer his arguments, and indeed to show that, so 

 far as our present knowledge goes, branchiomery and mesomery 

 are in good accord. 



