SUBPHYLUM VERTEBRATA 



319 



sternum ; the latter is composed of the girdles and the limbs. The girdles 

 are parts of the skeleton which more or less completely surround the 

 trunk and to which the skeletons of the limbs are articulated. 



The skeleton in the lowest vertebrates may be completely mem- 

 branous; in those next higher in structure it is made up largely of car- 

 tilage; while in those still higher it includes both cartilage and bone. 



5ku// 



Cervical 

 vertebrae 



Scapula 



Cranium 

 Orbit 



Mandible 



Clavicle 



Sternum 



Thoracic 

 vertebrae 



Lumbar 

 vertebrae 



Sacrum 



Metacarpals 



Humerus 

 Rib 



Pelvis 



Tibia 



/Metatarsals 



Fig. 213. — Human skeleton. 



Although some cartilage always remains, the proportion of bone increases 

 in the highest forms. In the embryological development of a vertebrate 

 the same order is to be observed, the skeleton passing through mem- 

 branous, cartilaginous, and bony stages. This is an illustration of the 

 biogenetic law. When the bones develop they either replace cartilage, 

 in which case they are known as cartilage hones, or they are formed in 

 fibrous tissue, frequently around cartilage, when they are known as 

 memhrane hones. 



