ORIGIN AND STRUCTURE OF THE HEAD 153 



somewhat less developed than the spinal ganglia behind 

 , it, though not so rudimentary as the ganglia in front of it. 

 Neither in Amniotes nor in Acanthias does it produce a 

 regular dorsal nerve. 



It will be better, however, to leave now FuRBRlNGER's 

 nomenclature and to replace it by the numbering of the 

 segments shown in plate I. 



The pro-otic and the anterior post-otic somites are no 

 longer to be recognized even in the earliest stages of 

 development. The first myotome probably develops from 

 what we may consider as the third post-otic somite. The 

 auditory vesicle acquires certain relations to the first of the 

 gillpouches behind the mouth. 



General conclusions. — From the foregoing consider- 

 ations the following conclusions result. Though in such 

 Selachians as Scyllium and Pristiurus the number of head seg- 

 ments happens to correspond with that of the visceral archs, 

 we must yet conclude, in opposition to Gegenbaur (1872), 

 that on the whole there is no relation between them. The 

 number of gill-slits may be considerably greater than that 

 of the segments of the skull, as is the case in Petromyzon, 

 or the skull may reach just as far as the last gill-slit, as in Uro- 

 delan Amphibia, or a greater or lesser number of post-branchial 

 segments may belong to it, as in Selachians and Amniotes. 

 In the first case the roots of the hypoglossus lie far behind 

 the skull, in the second case it is formed from the anterior 

 free ventral roots, in the third case a greater or. lesser 

 number of hypoglossus- roots become intracranial or occipital. 

 In the first case we shall speak of an incomplete palaeocranium, 

 in the second case of a complete palaeocranium, in the 

 third case of a neocranium. It depends on the number of 

 gill-slits whether the hypoglossus roots will lie far behind 

 the vagus or within the reach of its sphere of influence 

 which causes the dorsal ganglia behind it to atrophy. 

 That originally the hypoglossus has nothing to do with the 

 vagus is shown by Petromyzon. When the number of gill- 

 slits decreases, the h)^poglossus moves forward, i. e. its roots 

 now belong to segments more in front. This does not mean 

 to say, however, as FuRBRlNGER (1897, p. 440) assumed, 

 that the myotomes and their ventral roots themselves 

 move forward. Of such a "stetiges Vorriicken" there is no 

 question. The roots of the hypoglossus may by the process 

 mentioned above come so close to and under the partially 



