THE SEGMENTATION OF THE HEAD. 



145 



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. 



