MORPHOLOGY OF ANIMALS 



true vertebrates the notochord becomes surrounded by 

 cartilage, and the whole is then constricted into a series of 

 segments, the centra of the vertebrae; from these centra 

 cartilaginous arches grow dorsally around the spinal cord, 

 while other skeletal arches, the ribs^ surround the trunk 

 and later articulate with the vertebral column; finally 

 the ribs may be united ventrally, thus forming the ster- 

 num', these parts constitute the axial skeleton (Fig. 8). 

 The degree of development of notochord and vertebral 

 column in embryos and adults of different classes of verte- 

 brates is shown in the following table : 



In addition there is the skeleton of the head (the skull) 

 and that of the limbs (the appendicular skeleton). In the 

 lower vertebrates and in the embryos of all higher forms 

 the skull consists of a cartilaginous cranium partially sur- 

 rounding the brain, and of paired cartilaginous rods form- 

 ing the skeleton of the jaws and gill arches. In higher ver- 

 tebrates these cartilaginous elements undergo ossification, 



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