98 THE ANCESTRY OF VERTEBRATES 



the roots of the hypoglossus nerve in Amniotes. Several 

 authors have adhered to the view expressed by this double 

 nomenclature (hypoglossus, ventral vagus roots) but aban- 

 doned afteiwards by GeGENBAUR (1887, p 65) himself. 



Froriep's und FuRBRlNGER's v/eivs. — Others, however, 

 considered these nerves as ventral spinal nerves of which 

 the dorsal roots have been lost, a possibility alluded to 

 already by BALFOUR (1878, p. 205). This idea was worked 

 out for the hypoglossus of- Amniotes by PRORIEP (1882, p. 

 296) who demonstrated the presence of rudimentary dorsal 

 ganglia in the posterior roots of the hypoglossusjn embryos. 

 From this, and also from the discovery of well-developed 

 myotomes in the occipital region, he concluded that this 

 region must have belonged t^' the trunk, not to the primarily 

 unsegmented head region (cf. p. 65) to which region FRORlEP 

 also considers to belong the gill-slits and the vagus. According 

 to him in Amniotes an assimilation of vertebrae into the 

 cranium and a progressive backward extension of the 

 latter, as first suggested by StoHR ("der Schadel ist in 

 stetem kaudalen Vorriicken begriffen", 1881, p. 99), has occur- 

 red, by which originally post-branchial (i. e. not-cerebral, 

 but) spinal nerves were incorporated into the skull after 

 having lost their dorsal roots and ganglia, of which however 

 vestiges may be noted still in ontogeny especially above 

 the posterior roots. 



The same idea has been applied again, some time after- 

 wards, by FuRBRINGER (1897) — and before him ah eady 

 by others, as e.g. by GEGENBAUR (1887, p. 63) himself — to 

 Gegenbaur's ventral vagus roots in Elasmobranch^, in which 

 accordingly the skull has already assimilated a number of 

 vertebrae. The ventral vagus roots are considered as spinal 

 nerves that only secondarily have shifted forward into 

 the occipital region and under the vagus (vago-accessorius), 

 at the same time being incorporated into the cranium. 

 This view is based mainly on the circumstance that these 

 ventral roots, though anastomosing with the vagus, yet on 

 the whole join the ventral spinal roots behind them, thus 

 contributing to the formation of the cervico-brachial plexus, 

 and further on the fact, that here too more or less vestigial 

 dorsal roots to the posterior branches have been described 

 (VAN WYHE, 1886, p. 681, OSTROUMOFF. 1889, p. 364, 

 DOHRN, 1890, p. 82). The ventral vagus roots or hypo- 

 glossus (as formerly they were often designated) of Elasmo- 



