PRINCIPLES OF VERTEBRATE MORPHOLOGY 7 



As a rule, a series of two to ten or more metameres unites into a 

 fxmctional region and a concentration of these metameres occurs that 

 makes it difficult to discover the original segmental conditions in- 

 volved. Even the most primitive vertebrates exhibit wide departures 

 from the primitive metameric plan seen in some of the lower anne- 

 lids, and these modifications of the generalized segmental arrange- 

 ment become progressively more pronounced from the lowest to 

 the highest vertebrate classes. 



Cephalization 



One of the most significant advances over ancestral conditions 

 that the vertebrates have to their credit has to do with the evolution 

 of intelligence, and this is intimately bound up with the pronounced 

 specialization of the anterior metameres into a head, which is princi- 

 pally a brain and a group of sense organs. This foreshortening, con- 

 densation, and specialization of the anterior metameres has been 

 called cephalization. The course of evolution from the lowest to the 

 highest vertebrates has been one of more and more pronounced 

 cephalization involving a progressively larger number of metameres, 

 an increased dominance of the head over the rest of the body, and 

 closer integration of all the functions. The climax of the process of 

 cephalization is Man, who is essentially a dominating intelhgence 

 and a set of accessory organs. Various attempts have been made to 

 decipher the highly modified metamerism of the vertebrate head. 

 One of these attempts gave rise to the classic "Vertebral Theory " 

 of the skull by Goethe and by Oken. It was thought that the skull 

 was made up of a modified series of vertebral units and that an analysis 

 of these elements would give the number of metameres. This theory 

 was proven by Huxley to be untenable. The cranial nerves have been 

 taken as evidences of metamerism, but the old arrangement of ten 

 cranial nerves is no longer taken to indicate ten metameres, since some 

 of these nerves are the motor and others the sensory components of 

 single metameric divisions of the neuron. Perhaps the most reliable 

 index of the number of metameres in the head is seen in the mesoblas- 

 tic somites of some of the fishes and cyclostomes. There are but three 

 somites in front of the auditory capsule, which forms a convenient 

 landmark in that it seems to mark off the original head from the post- 

 cephahc region. Possibly then the primitive craniate had a head 

 of three metameres, a number characteristic of the larva of Balano- 



