630 EMBRYOLOGY. 



produce the evidence that the primordial eraniv/m has arisen hy fusion 

 from a number of segments equivalent to vertehrm. Instead of the 

 Oken-Goethe vertebral theory he propounds the segmental theory of 

 the skull, as I suggest the doctrine of Gegenbaur be called. 



Gegenbaur proceeds from the correct conception that the segmen- 

 tation of a region of the body is recognisable not only in the meta- 

 merism of the vertebral column, but also in many other structures — 

 in the method of the arrangement of the chief nerve-trunks, and in 

 the ventral arch-structures attached to the axial skeleton. He 

 investigates, accordingly, the cranial nerves of the Selachians, and 

 arrives at the conclusion that, mth the exception of the olfactory 

 and optic nerves, which are metamorphosed parts of the brain itself, 

 they deport themselves like spinal nerves both in their origin and 

 their peripheral distribution. He determines that there are nine 

 pairs of them ; and therefore concludes that the portion of the head- 

 skeleton which is traversed by the nine segmentally arranged cranial 

 nerves must be equivalent to nine vertebral segments, and that it 

 must have arisen by their very early fusion. 



The visceral skeleton of Selachians is regarded by Gegenbaur 

 from the same instructive point of view. He discerns in the 

 maxillary, hyoid, and branchial arches skeletal elements which are 

 represented in the vertebral column by the ribs. 



Inasmuch as a vertebral segment belongs to each pair of ribs, a 

 similar relation is also assumed as the original arrangement for the 

 visceral arches. Thus this method of considering the question leads 

 to the same result : that the primordial cranium — since at least nine 

 visceral arches belong to it as ventral arch-structures — has been 

 produced from at least nine segments. 



Such an origin Gegenbaur accepts for the posterior chorda- 

 traversed region of the skull only, in which alone the emerging 

 nerves agree with spinal nerves. He therefore distinguishes this as 

 vertebral from the anterior or non-vertebral portion, which does not 

 allow the recognition of any segmentation, and which begins in front 

 of the anterior end of the chorda. He interprets the latter as a new 

 formation which has been established by the enlargement in front of 

 the vertebral part of the skull. 



Gegenbaur explains the great differences which exist between. 



skull and vertebral column as adaptations, partly to the enormous 



development of the brain, partly to the sensory organs of the head^ 



which are received into pits and cavities of the primordial cranium. 



Since the time when Gegenbaur with keen discrimination pro- 



