INTERNAL ORGANS 163 



to develop, much of the cartilage disappears, and the skull 

 gradually assumes the adult form. It is of interest to note that 

 the dermal bones have become so much an essential part of 

 the cranium that they have in some way inhibited the growth 

 of cartilage in the regions which they will finally occupy, and 

 during the early stages of development the brain is protected 

 merely by connective tissue in these regions. 



The series of cartilages or bones constituting the axis of 

 the body, designed to afford protection for the delicate spinal 

 cord and for certain blood-vessels running from head to tail, 

 is known as the vertebral column or backbone (Fig. 65), and 

 the separate elements are spoken of as vertebrae. It is very 

 probable that this was the first part of the skeleton to be evolved, 

 and, indeed, there is good reason for believing that the skull 

 itself arose primarily through the fusion and modification of 

 some of the anterior vertebrae. In the developing embryo of 

 any fish the first part of the skeleton to make its appearance is 

 not the proper backbone but an unjointed rod of gelatinous 

 tissue, the notochord, running along the axis of the body and 

 ending in front between the rudiments of the cartilages forming 

 the floor of the cranium. The ancestors of the fishes probably 

 retained this simple axial rod throughout life, but with the 

 ever-growing need for some protection for the spinal cord, as 

 well as for a centre for the attachment of the body muscles, 

 the vertebral column was developed round it. During the 

 development of the individual fish the different vertebrae 

 arise as rings of cartilage which grow round the notochord and 

 gradually constrict it. Later on, additional pieces of cartilage 

 grow up to surround the spinal cord, and others are developed 

 below as a protection for the main artery and vein. In all 

 the higher fishes the notochord disappears altogether in the 

 adult stage. 



In the Lamprey [Petromyzon) the vertebral column is very 

 simple, the notochord persisting in the adult, and merely 

 supporting a series of isolated cartilages on either side of the 

 spinal cord. In the Selachians it is more complicated, but is 

 still composed entirely of cartilage. Each vertebra is a complex 

 structure made up of a number of pieces firmly joined together. 

 The names of all these elements need not be mentioned here, 

 and it will suffice to point out that the body of each vertebra, 

 the centrum, takes the form of a ring of cartilage, which is 

 hollow in front and behind Hke a dice-box: on its upper and 

 lower edges this bears an arch of cartilage, the upper or neural 



