112 THE ANCESTRY OF VERTEBRATES 



different groups; just as much can this be the case with 

 ihe segments to which the branchial and the lumbal plexus 

 belong. In the year 1879 FuRBRINGER (p. 389), as a result 

 of his researches on the plexus brachialis and lumbosacralis, 

 came to the conclusion that these plexus are not bound to 

 definite myotomes, but that their situation and extent depend 

 on the situation and development of the limbs they innervate. 

 If the limb be strongly developed the number of segments 

 participating in supplying this with muscles and nerves may 

 increase and, in the opposite case, it may decrease, though 

 no intercalation of new segments, or falling out of segments 

 formerly present, can be assumed to account for this. In 

 the same way the shifting forwards or backwards of the 

 limbs is not the result of a moving of corresponding 

 segments but adjoining segments take over the task of the 

 old ones which now come free. GOODRICH (1914) has 

 recently contributed a very interesting artick on this subject 

 and has shown plainly that there is no primary relation 

 between homology and metamerism. We can speak only 

 of a regional homology, the homology of a region or 

 segmental level which may move caudad and rostrad 

 •and may extend over a grCftter or lesser number of 

 segments. 



The anterior limit of the hypoglossus-level is determined 

 by the situation of the last gill-slit, i. e. by 'the number 

 ot gill-slits or the extent of the branchial level. From this 

 and from the backward extent of the sku.ll depends whether 

 the hypoglossus will lie far behind the cranio-vertebral limit 

 (Petromyzon,) whether its anterior limit will nearly coincide 

 with it (Amphibians and several Elasmobranchs), or whether 

 the hypoglossus will be for a lesser or greater part intra- 

 cranial (other Selachians, Amniotes). It also depends on the 

 situation of the last gill-slit whether the hypoglossus-region 

 will be" found far behind the vagus and beyond its sphere 

 of influence which causes the dorsal roots and spinal ganglia 

 behind it to atrophy {Petromyzon) or whether it will reach 

 with its anterior end into the sphere of influence of the 

 vagus (Selachians, Amniotes). 



If then at least part of the occipital myotomes and their ner- 

 ves are to be considered as belonging primarily to the branchial 

 region and, as argued before, FRORlEF's distinction between a 

 cerebral unsegmenied and a spinal region of the cranium must 

 be rejected, then our conception of the occipital region becomes 



