EVOLUTION OF THE NOTOCHORD. 2QI 



(Fig. 263, a). This " gelatinous nucleus " of the elastic ver- 

 tebral disc becomes less sharply defined, but persists 

 throughout life in all Mammals, while in Birds and Rep- 

 tiles, even the last remnant of the notochord vanishes. 



FTG. 262. A brenst-vertebraof the same embryo in lateral cross-section: 

 cv, cartilaginous vertebral bodies ; ch, notochord ; pr, square process ; 

 a, vertebral arch (upper) ; c, upper end of rib (lower arch). (After 

 Koelliker.) 



FIG. 263. Intervertebral disc of new-born child in cross-section : 

 a, remnant of the notochord. (After Koelliker.) 



When the cartilaginous vertebrae afterwards ossify, the first 

 deposit of bone-substance (the first "bone-nucleus") in 

 the vertebral bodies is formed immediately round the rem- 

 nant of the notochord, and soon completely displaces the 

 latter. A special bone kernel or nucleus is then formed in 

 each half of the cartilaginous vertebral arch. It is not till 

 after birth that the ossification progresses so far that the 

 three bone-nuclei approach each other. The two bony 

 halves of the arch unite during the first year, but it is not 

 till much later, till between the eighth and the twelfth 

 year, that they unite with the bony vertebral body. 



The bony skull (cranium), which must be regarded as 



