114 THE ANCESTRY OF VERTEBRATES 



we might rather expect that the backward extension of 

 the branchial baslcet into the trunk region, which pushes the 

 anterior nerves aside and thus favours the formation of the 

 cervico-brachial plexus in Elasmobranchs, would pull the 

 roots of these nerves somewhat in a backward direction. 

 Would not then rather the old conception, defended by His 

 ( 1 887, p. 401 , 414, 423) against Froriep, be the right one : that 

 the three hypoglossus-roots of Amniotes, being indeed nothing 

 else than the ventral occipital nerves of Elasmobranchs, 

 may be termed, like the latter, ventral vagus roots, not a, 

 b, c but something like x, y, z, which already in Elasmo- 

 branchs contribute to the innervation of the hypobranchial 

 musculature and to which they show such an undeniable 

 resemblance in their situation, their peripheral distribution 

 and in the reduction of their dorsal roots in rostro-caudal 

 direction? "Die Trennung des Hypoglossusgebietes vom 

 Accessorius- und Vagusgebiet", says His (1887), "halte ich 

 fiir eine durchaus kiinstliche und ich berufe mich in der 

 Hinsicht auf die Beobachtung, wonach der Hypoglossuskern 

 auf eine lange Strecke den motorischen Kernen von Acces- 

 sorius, Vagus und selbst Glossopharyngeus parallel laufen". 

 (p. 401). "Ein einziger Blick auf die Disposition der Ur- 

 sprungsherde genijgt, um erkennen zu lassen, wie willkiirlich 

 es ist, das Hypoglossusgebiet vom Vagusgebiete trennen zu 

 wollen und jenes dem Rumpfe, dieses dem Kopfe zuzu- 

 theilen" (p. 414). 



FRORIEP (1905, p. 120) himself does not join FuRBRlNGER 

 in his deductions concerning the hypoglossus but feels 

 inclined to homologize the occipital somites of Am- 

 niotes to those of Selachians with which they agree in 

 so many points. In this connection he still mentions the 

 fact that the anterior end of the pronephros in Amniotes as 

 well as" in Selachians is found as a rule in the third somite 

 behind the cranio-vertebral limit. Truly, too great weight 

 should not be attached to this circumstance, since the 

 situation of the first pronephric funnel is no more bound 

 to a definite segment than that of the plexus of the limbs. 



Amphibian and Selachian skull. — As we have seen, the 

 whole distinction of a proto- and an auximetameric neo- 

 cranium and of occipital and occipito-spinal nerves, as far 

 as concerns Amniotes, is based on FuRBRlNGER's assumption 

 that the skull of the Amphibians is equal in length to that 

 of Elasmobranchs and that it is a protometameric neocra- 



