154 THE ANCESTRY OF VERTEBRATES 



polymeric vagus that they may be termed with some right 

 ventral vagus roots, .as has been done by GeqENBAUR. 

 Better still would it be, no doubt, to speak of ventral 

 accessorius roots or vago-accessorius roots, for the acces- 

 sorius originates in closest connection with the anterior 

 rudimentary ganglia behind that of the primary vagus. In 

 Selachians it still forms part of the vagus which here 

 innervates also the Musculus trapezius, one of the muscles 

 in the branchial region which originate from the lateral 

 plate and are innervated by the dorsal nerves of the head. 

 The strong development of this muscle in higher Vertebrates 

 causes the accessorius to split off from the vagus. 



Hypoglossus homologous in Vertebrates ? — Is the hypo- 

 glossus homologous in the different groups of Vertebrates ? 

 1 think this question must be answered in the affirmative. 

 Truly, its roots belong to segments bearing different numbers 

 in different groups and still more do their relations to 

 the skull vary. The first circumstance, however, as has 

 been argued recently exhaustively by GOODRICH (1914), 

 is no hindrance to considering structures as homolo- 

 gous. On the contrary, homology is quite independent 

 from metameric segmentation and only secondarily can there 

 be established a more or. less fixed relation between 

 both, especially in forms with a restricted number of 

 segments, and at the anterior end of the body, e.g. in 

 the anterior segments of the head and their organs in 

 Craniates. Were we to make homology dependent upon 

 the number of the segments from which the organs arise, 

 then there could be no question of an homology of the 

 paired limbs among fishes and tetrapods, as the segments 

 from which they arise are by no means the same and 

 subject to considerable variation both in number and in 

 situation. The same holds good e.g. for the pronephros. In 

 Urodelan Amphibians the first pronephric funnel is found 

 in the second segment behind the skull (FIELD, 1891, 

 p. 261, Goodrich, 1911, p. 112), i.e. in segment nr. 7 

 which in Selachians and Amniotes belongs to the skull. 

 In the latter groups the first funnel is found as a rule 

 in the third segment behind the skull, i.e. in segment 

 about nr. 11 (FRORIEP, 1905, p. 119). Nobody will doubt 

 the homology of these organs. The "segmental level" to 

 which they belong may move backwards and forwards, may 

 extend over a greater or over a lesser number of segments 



