2 [2 Journal of Comparative Neurology and Psychology. 



been given for the disappearance of a gill in the middle of the 

 series. The only indication of it is the connection of somites 



3 and 4 with the hyoid arch in selachians. This would indicate 

 that the lost gill slit must have cleft the present hyoid arch, as 

 V. WijHE thought. Since it is impossible to explain the nerve 

 relations on this supposition, Neal (96) suggests that the lost 

 branchial segment must have been "anterior to the present 

 hyomandibular cleft," corresponding in position to the blank 

 neuromerc between Nn. V and VII. Even so, the praetrematic 

 and posttrematic rami of VII are made to embrace two gills, 

 the present hyomandibular cleft and the lost one; unless, indeed, 



it is supposed that the lost cleft divided the mandibular arch 

 between the mandibularis V and the mandibularis VII nerves. 

 On the ground of mesoderm, however, it is the hyoid that is 

 supposed to have been cleft. Braus' Figs. 5 and 6, PI. XXI, 

 indicate that the connection of somite 4 with the hyoid arch is 

 secondary. One very important feature of this blank neuro- 

 mere which has not been noticed by previous authors is that it 

 is not the same in all vertebrates, but shifts caudad in higher 

 forms just as the trigeminus does. This does not seem to have , 

 any relation to a lost gill in the hyoid region. Since the hy- 

 pothesis of a lost gill was brought forward to explain the nerve 

 relations, it loses its interest if these relations are satisfactorily 

 explained on some other basis. 



For the explanation of the blank neuromeres the most sug- 

 gestive facts are found in the position of the auditory vesicle 

 and in the position of the nerve centers related to the cranial 

 nerves. In Petromyzon (Koltzoff's figures), the auditory 

 vesicle when first formed lies oposite neuromere vii and the 

 facial ganglion passes down mesial to its caudal portion. In the 

 adult (i, 68) N. VII passes through the mesial wall of the audi- 

 tory capsule and out through the ventro-cephalic angle. There 

 has apparently been some shifting back of the auditory sac dur- 

 ing the ontogeny of Petromyzon. In selachians and all Gna- 

 thostomes the auditory sac is situated farther back, about 

 opposite the tenth or eleventh neuromeres. During the 

 early development in Sgiialus acantJiias (125) the auditory pit 



