356 COMPARATIVE ZOOLOGY OF CHORDATES 



Above the mandibular somite is the ganglion of the tri- 

 geminal nerve which is the dorsal root of the 2nd segment, 

 corresponding to the trochlear. The hyoid somite lies under 

 the ganglion of the facial nerve, which is accordingly the dorsal 

 root of the 3rd segment, corresponding to the abducens. It 

 is possible, therefore, to recognise three prootic segments. 



The glossopharyngeal nerve is the dorsal root of the 4th 

 segment, overlying the 4th somite. The vagus represents 

 parts of four dorsal roots joined together, and it corresponds 

 to the 5th to 8th segments. The ventral roots of the 4th and 

 5th segments which disappear in the dogfish, are present in 

 Petromyzon. The ventral roots of the 6th and following 

 segments are present in the dogfish, innervating the anterior 

 myotomes, and contributing to the hypoglossal nerve which 

 accompanies the growth downwards and forwards of portions 

 of the myotomes to form the hypoglossal muscles. 



So far, then, the only difference between the head and trunk- 

 regions is that in the former, the dorsal and ventral nerve-roots 

 remain distinct from one another, and that in the three prootic 

 somites the ganglia of the dorsal roots lie outside the little 

 somites, instead of median to them as in the trunk. 



It is now time to turn to the gill-slits, which arise as out- 

 growths from the pharynx on each side, and connect with the 

 ectoderm. The gill-slits are formed at a level below that of 

 the somites, in the region of the lateral plate, or unsegmented 

 mesoderm. The connexion of the endoderm of the pharynx 

 with the ectoderm in the formation of the gill-slits necessarily 

 obliterates to the mesoderm in places and confines it to the bars 

 between the gill-slits. These are the gill-bars (gill-arches, or 

 visceral arches). The remnants of the splanchnocoelic cavity 

 in this region are restricted to the cavities in the gill-bars 

 (as in the primary gill-bars of Amphioxus). 



Now, down each of these gill-bars or visceral arches there 

 passes a large branch of a dorsal nerve-root. The most 

 anterior visceral slit is the spiracle, and separating it from the 

 mouth is the mandibular arch (or 1st visceral arch) down 

 which the trigeminal nerve passes. Between the spiracle and 

 the 2nd visceral slit (1st gill-slit) is the hyoid arch (or 2nd 



