OUR ANCIENT RELATIVES 



succession from the air-breathing fishes of pre- 

 Devonian times. 



However, we wish to go even back of that and 

 are curious to know why animals ever acquired a 

 skull at all. The "basic patent" for the strength- 

 ening of all skeletal parts is the bone-cell, which 

 invades both the skin covering the head, where 

 it forms "derm bones," and the underlying carti- 

 lage or braincase; everywhere it deposits phos- 

 phate of lime and other salts, thereby greatly 

 stiffening the skin and strengthening the brain-box. 



The skull of all vertebrates above the sharks is 

 a complex bony structure consisting of an outer 

 shell, or dermocranium, originally derived from 

 the many-layered skin, and an inner skull, or 

 endocranium, derived from the cartilaginous brain- 

 trough and its associated three pairs of capsules 

 for the nose, eyes and inner ears. 



The same kind of cells surround the elastic 



notochord or primitive axial rod, and deposit the 



bony tissue along certain tracts between the tough 



membranes that separate the muscle segments. 



In this way rods called ribs are produced as well as 



the bony arches above the notochord. All this 



results in a strong framework, which supports the 



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