HEAD OP SQUALUS 471 



to three or more segments, but even going by topographical 

 relations alone it is not unreasonable to regard the abducens 

 as being the genuine third ventral root. At any rate I do not 

 see that the condition of the abducens furthers the assumption 

 that a gill-slit has been lost. If Neal's contention is true the 

 whole question of the eye muscles innervation and segmenta- 

 tion will require revision. 



Lastly, one more train of thought has been brought to 

 bear on the supposedly lost organs, and that is the ques- 

 tion of the relation of neuromeres to the other segmental 

 structures. 



Neal (11) describes seven neuromeres, of which the first 

 corresponds to the anterior head cavities with the olfactory as 

 its dorsal nerve : this is, of course, assuming that the anterior 

 head cavities have the value of a somite anterior to the pre- 

 mandibular somites. To the second neuromere correspond the 

 ramus ophthalmicus profundus premandibular somite and 

 oculomotor. The third or cerebellar neuromere later under- 

 goes subdivision (which Neal regards as secondary) and to it 

 belong trigeminal, patheticus, and mandibular somite. The 

 next dorsal root, the facial, arises from the fifth neuromere, 

 and this led to the idea that the nerve of the fourth neuromere 

 (which has none) has disappeared and that it was this nerve 

 which was related to the lost gill-slit. To the sixth and seventh 

 neuromeres belong the glossopharyngeal and vagus, though 

 their topographical correspondence has been lost. The glosso- 

 pharyngeal appears to arise from the seventh neuromere, and the 

 vagus behind it ; but this is explained as being due to the pres- 

 sure of the auditory sac and relative shifting of the elements 

 of the neural crest and neural tube. These relations are shown 

 in the embryo (6 mm.) reconstructed in Text-fig. 13. 



If it be granted that neuromeres have a primary segmental 

 value, then it may be said that there is one neuromere too many 

 overlying the hyoid arch ; but that such a segmental value 

 exists remains to be proved. To start with it rests on the 

 assumption that the anterior head cavities represent a somite. 

 ^ These are present only in Galeus and Squalus, but in Amia, 



