CHAPTER V 



THE RELATIONSHIP OF EXISTING ORGANISMS 



(Continued) 



3. COMPARATIVE ANATOMY OF VERTEBRATES 



The preceding consideration of embryological relationship has 

 necessarily touched upon some of the salient features of compara- 

 tive anatomy. The transition of skeletal development, the struc- 

 ture of the heart in the several classes, development of the aortic 

 arches and the venous system may as well be treated in one field 

 as in the other, since they cannot be made clear without reference 

 to both. Comparative anatomy discloses, however, a great many 

 details of homology without reference to development. Many 

 vestigial structures which anatomists have found in man, for 

 example, are shown to have the same relations as functional 

 structures in the bodies of other animals, while the functionally 

 active systems are built up of the same tissues and organs, arranged 

 according to the same plan. While the similarity of functional 

 parts is in itself significant, the occurrence of vestigial organs, 

 particularly those which are found only in occasional individuals, 

 is doubly so. We may content ourselves with necessity as a 

 reason for the presence of useful organs, but obviously useless 

 structures can be explained only on a very different basis. 



The Skull. In the skulls of vertebrates from the fishes to the 

 mammals a large number of bones are found, some present in all 

 forms, some in only the lower forms. We have already noted that 

 these are of two different kinds, those originating in cartilage as 

 parts of the chondrocranium and those which develop directly 

 from the embryonic mesoderm. The latter are of particular 

 interest at this point because of the completeness of their history 

 as shown by existing forms. The chondrocranium, or primordial 

 skull, is well developed in the elasmobranch fishes as a supporting 

 structure extending forward from the spinal column beneath the 

 brain, which it does not enclose dorsally or anteriorly except by 

 the development of secondary membranous structures. The same 

 primordial skull develops in forms above the elasmobranchs, but 



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