PAIRED PINS WITH THEIR NERVES, IN LEPIDOSIREN AND PROTOPTERUS. 615 



overlap the posterior part of the anterior one (branchial region). The occipital 

 nerves belong, of course, to the occipital myotomes (ccenogenetic), and the vagus, 

 which is formed by the coalescence at their proximal ends of the splanchnic fibres of 

 the segmental nerves belonging to the posterior palingenetic head segments (of which 

 only the splanchnic or branchial portions now remain), as belonging to the head proper. 



This view has been maintained by Gegenbaur, Froriep, Furbringer, Braus, etc. 



Except for the fact that the posterior gill clefts are formed ventral to the occipital 

 myotomes, and in Scyllium and Pristiurus, according to van Wijhe, the third visceral 

 arch includes a portion of the lateral plate mesoderm while this is still in connection 

 with the myotomes — i.e. before relative shifting is likely to have taken place ontogen- 

 etically — the preponderance of evidence seems to be in favour of the second view. 

 The origin of the mesoderm of the branchial arches — whether from the same segments 

 as the myotomes under which they are situated or not — is obviously of the greatest 

 importance in deciding this question. 



The facts in Lepidosiren and Protopterus are as follows : — As already mentioned, 

 the segmented trunk mesoderm (occipital myotomes) stops short some distance behind 

 the auditory vesicle. 



The first gill outgrowth is pro-otic. 



The second gill outgrowth is partly pro-otic, partly hypotic. 



The third gill outgrowth is at first metotic, but, by backward extension of the 

 auditory vesicle, very soon comes to be hypotic. 



The fourth, fifth, and sixth outgrowths are metotic. 



The visceral arches are constituted as follows : — 



One and two include mesothelium connecting lateral plate with unsegmented 

 pro-otic mesoderm. 



Three includes mesothelium connecting lateral plate with unsegmented hypotic 

 mesoderm. 



Four includes mesothelium connecting lateral plate with unsegmented metotic 

 (subsequently hypotic) mesoderm. 



Five, six, and seven are formed after connection between the lateral plate and 

 somatic mesoderm is broken through, and include only loose mesenchyme. 

 They are formed in the region of the occipital myotomes (x, y, z). 



Owing to the fact that these last visceral arches include loose mesenchyme only, 

 it is not necessary to suppose that they belong genetically to the myotomes below 

 which they are situated. 



Lepidosiren and Protopterus therefore give no embryological evidence in favour of 

 the first of the alternative views as to the nature of van Wijhe's metotic head somites. 

 Even in such cases as Scyllium and Pristiurus, where the third gill outgrowth cuts off 

 lateral plate mesothelium still in connection with an occipital myotome, it is not 

 necessary to suppose that the two structures originally belonged to the same segment. 

 That lateral plate structures may in ontogeny appear under, and attached to, somatic 



