THE VERTEBRATA 



139 



It runs from the region known as the mid-brain, 

 to the end of the tail, and lies throughout just 

 beneath the spinal 

 cord. Whatever its 

 original use in the 



A B 



FIG. 41. .4, The Notochord of Vertebrates. Section, considerably 

 magnified, through the middle of an embryo one inch long, of 

 Acanthias, one of the Spiny Dog-fishes allied to the sharks, i, 

 Section through Spinal Cord ; 2, Section through Notochord ; 

 below it lies a bean-shaped space, which is a section through a 

 large blood-vessel ; sk, epiblast or skin ; me, mesoblast or mid- 

 dle layer of the body ; the dots represent the nuclei of its trans- 

 parent cells. The intestine, t, lined with hypoblast, is traversed 

 by a spiral valve, and surrounded by the horse-shoe shaped 

 body-cavity. B, Diagram indicating the position of the Noto- 

 chord in the vertebra of an adult Common Dog-fish (Scyllium 

 canicula). i, " Neural arch " of the vertebra, consisting of 

 processes of bone enclosing the central nervous system, or spinal 

 cord ; 2, bony centrum of the vertebra, hollowed out into a cup, 

 in which lies a soft pad, the remains of the notochord. 



