XII.] 



THE SKULL. 



401 



the intervertebral regions it remains relatively uncon- 

 stricted (Figs. 134! and 135 c) and after undergoing 

 certain histological changes remains through life as part 

 of the nucleus pulposus in the axis of the intervertebral 

 ligaments. There is also a slight swelling of the noto- 

 chord near the two extremities of each vertebra (Fig. 

 135 c' and c"). 



In the persistent vertebral constriction of the notochord 

 Mammals retain a more primitive and piscine mode of formation 

 of the vertebral column than the majority either of the Eeptilia 

 or Amphibia. 



Fio. 134. 



Longitudinal Section THaonsH the Vertebral Coltjmn 

 or AN Eight Webkb' Human Embryo in the Tho- 

 racic Region. (From KoUiker.) 



V. cartilaginous vertebral body ; li. intervertebral Mgament ; 

 ch. notochord. 



The skull. Excepting in the absence of the inter- 

 orbital plate, the early development of the Mamma- 

 lian cranium resembles in all essential points that of 

 Aves, to our account of which on p. 235 et seq. we refer 

 the reader 



F. & B. 26 



