1017 



vagus (vago-accessorius) or not (Gegenbaur). Tn Pelromyzon e.g. this 

 is not the case. 



On the number of gill-slits and on the backward extension of the 

 skull depends whether the anterior hjpoglossus-roots are incorporated 

 into the skull as occipital nerves. In Petromyzon the skull is short 

 and the number of gill-slits great : the hypoglossus accordingly lies 

 far behind the skull. Directlj- behind the skull we find the hypo- 

 glossus roots in Amphibians and most rays; partly in the skidl and 

 partly behind it in sharks; for the greater part in the skull in 

 Amniotes, wholly in the skull in Chondrostei, where moreover the 

 plexus brachialis may have been incorporated into it, as is the case 

 in Aclpenser CFürbringer, 1827, p. 457). 



From the above considerations the following conclusions may 

 be drawn : 



1. Froriep's (1882 — 1887) sub-division of the head of Vertebrates 

 into a primarily unsegmented "cerebral" part, comprising besides 

 eye and nose also the auditory vesicles and the gill-slits, and a 

 segmented "spinal" part (regio occipitalis) is false. Gegenbaur's 

 division of the skull into a prae-chordal "evertebral" and a chordal 

 "vertebral" part is the right one, though the anterior part of the 

 latter, as far as the occipital arch, has not formed from vertebrae, 

 but has originated simultaneously with the latter (cf. Petromyzon, 

 Gegenbaur, 1887, p. 77, van Wijhe, 1889). Branchiomerism and 

 raesomerism correspond. 



A primarily unsegmented head mesobiast (Froriep, 1887, "Urmeso- 

 derm" of de Lange, 1913, p. 250), in which we could speak only 

 of branchiomerism, does not exist; the prostomium no longer contains 

 primordial mesoderm (cf. Amphioxus and the"proammion" of Amniotes). 



2. Froriep's (L882 etc.) conception of a secondary invasion of 

 trunk segments into the primarily unsegmented "cerebral" head and 

 Fürbringer's (J 897, p. 440) opinion on a "stetiges Vorriicken" and 

 breaking down of these myotomes with their ventral roots in the 

 occipital region are false. Froriep's argument that rudinjentary dorsal 

 roots discovered by him belonging to these myotomes would indicate 

 that they cannot belong to the vagus and must be of post- 

 branchial origin, loses its value by the conception of the vagus 

 as a partially polymeric nerve, which would lead us naturally to 

 expect just such rudimentary dorsal roots. The argument of Für- 

 bkinger, that the occipital nerves of the sharks unite with the 

 anterior free spinal nerves to form the plexus cervicalis, is wholly 

 deprived of its value by the above considerations. Froriep and 

 Fürbringer, not making a difference between primarily and 



