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COMPARATIVE ANATOMY 



The hypoglossus is evidently a spinal nerve which has become cranial in 

 amniotes. It is a reasonable conclusion that originally cranial and spinal 

 nerves were similar and serially homologous. Amphioxus presents the 

 living evidence that a serially homologous segmentation extended through- 

 out the entire length of the chordate ancestor from which vertebrates 

 evolved. The differences between head and trunk, therefore, are coeno- 

 genetic, and are due largely to the concentration of the major senses 

 in the head and the high degree of differentiation of cranial organs. 



All who have studied head metamerism agree that the head is seg- 

 mented. Some, however, have concluded that the prootic region is a 

 region sui generis. Yet the great majority compare prootic and metaotic 

 segments with those of the trunk. Some disagreement appears in esti- 

 mates of the total number of segments. This is not surprising, since the 

 number of occipital segments varies in different classes. That vertebrae 

 have been added to the skull is shown by the fact that the accessory and 

 hypoglossal nerves are cranial in amniotes but spinal in anamnia. 



SUMMARY OF THE EVIDENCE 



The resemblances and differences between head and trunk metameres 

 may be summarized thus: 



I. Resemblances 



Head 

 Coelom 

 Myotome 

 Sclerotome 



Somatic efferent nerve 

 Visceral efferent nerve 

 Somatic afferent nerve 

 Visceral afferent nerve 

 Sympathetic 

 Neuromere 

 Vertebrae (occipital) 



Aortic arch 

 Visceral cleft 

 Epibranchial sense organ 

 Visceral skeleton 

 None 



II. Differences 



Trunk 

 Coelom 

 Myotome 

 Sclerotome 



Somatic efferent nerve 

 Visceral efferent nerve 

 Somatic afferent nerve 

 Visceral afferent nerve 

 Sympathetic 

 Neuromere 

 Vertebrae 



None 

 None 

 None 

 None 

 Kidney tubule 



It is, therefore, evident that head and trunk segments in vertebrates 

 are fundamentally similar. To prove this comparability it is not necessary 

 to demonstrate that there are no differences in regions which are so 

 highly specialized, since the observed differences might arise through 

 adaptive modification of parts originally alike. The most convincing 

 evidence of head metamerism is that manifested in the mesodermal 

 somites of anamniote embryos. 



