HEAD OP SQUA.LUS 467 



arch so that it is not included in the cranium. Its fibres join 

 those of the spinal ganglion to form the first mixed nerve, 

 branches of which I have traced to the tenth somite. Since 

 the eighth somite is the last of the vagus segments the ninth 

 is morphologically the first spinal or post-vagal (in the case of 

 Squalus included in the skull), and the tenth, which in Squalus 

 forms the first mixed root, is really the second spinal or post-vagal. 

 Eudimentary dorsal ganglia are present belonging to the seventh, 

 eighth, and ninth somites (Text-fig. 9). In Squalus, therefore, 

 there are nine segments included in the skull. Hoffmann (9), 

 Sewertzoff (17), and others state that there are ten, but since they 

 adopt Van Wijhe's somites, and like him intercalate a somite be- 

 tween those related to the fifth and seventh nerves, the number 

 of their somites from the third backwards are the same as those 

 of Balfour, plus one. Hence their results and mine are really 

 in accordance since we both regard the same segment as being 

 the last one included in the skull, though the numbers attri- 

 buted to it are different. 



As compared with Scyllium Squalus has two more seg- 

 ments included in the skull. But it is interesting to note that in 

 both forms it is the tenth segment (second spinal) which gives 

 rise to the first mixed nerve. It is another proof that homology 

 does not depend on numerical correspondence (Goodrich, 6). 



Discussion. 



The acceptance of Van Wijhe's scheme of segmentation 

 renders it necessary that two somites, the third and fourth, 

 should be associated with a single cleft and visceral arch : 

 the spiracle and hyoid arch. 



There is in this region one dorsal nerve, the facial, and this 

 Van Wijhe regards as double and representing elements 

 belonging to the third and fourth somites. The hyoid arch he 

 assigns to the fourth somite, and in order to account for the 

 third he assumes that a visceral cleft and arch have been lost. 

 We shall return to this assumption later. Further, he regards 

 the somites from the fourth to the eighth as related to the 

 visceral arch and dorsal root lying in front of them. Now the 



