STRUCTURE OF THE VERTEBRATES 171 



selective value upon locomotion, and at the same time degen- 

 erative changes are not necessarily fatal. 



Due to the interdependence of the skeletal and muscular sys- 

 tems the bones may be modified by mutations in other tissues. 

 An inherent degeneration of the muscles would undoubtedly af- 

 fect the bones. The degeneration of the embryonic coracoid of 

 the marsupial can be interpreted in this way, for there are no 

 muscles attached to them. Conversely, the degeneration of a 

 bone will cause developmental abnormalities in the muscles. The 

 thumbless monkeys have all the thumb muscles, although they 

 are vestigial and almost microscopic in some specimens. The 

 interdependence of tissues is discussed in Chapter XIX. 



I. Development of the Skeleton 



The two developmental types of bone (dermal and replace- 

 ment) have been defined. Either may give rise to the two ana- 

 tomical types, long and flat bones. In the higher vertebrates few 

 of the long bones are derived from dermal structures, these be- 

 ing limited to the dermal ribs and a part of the clavicle in a 

 number of amphibia and reptiles. Flat bones include the body 

 plates of the lower vertebrates, and the bones composing the 

 skull of all bony vertebrates. Thus, the supra-occipital is flat, 

 and composed in part of both replacement and dermal bone. 



In addition to the above, bone is classified by histological 

 structure into compact, or hard bone, and cancellous, or spongy. 

 If a long bone is hemisected it will be seen that the head and 

 the inner part of the shaft are made of coarse fibers which 

 divide and anastomose, forming a spongy substance with many 

 air spaces between the fibers. The heavy part of the shaft is a 

 solid, compact structure. The flat bones are formed of two 

 layers of compact bone, with an inner layer of spongy bone. 

 The following classification will make this clear. 



Classification of Bone 

 Embryological Anatomical Histological 



1. DennalzziIIII~ \ Long^_ ^Compact 



2. RoploppTYiPTif. % Flat ^ Cancellous 



