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 thai} tfye majority either of the Reptilia 

 or Amphibia. 



FIG. 134. 



LONGITUDINAL SECTION THROUGH THE VERTEBRAL COLUMN 

 OF AN EIGHT WEEKS' HUMAN EMBRYO IN THE THO- 

 RACIC REGION. (From Kolliker.) 



v. cartilaginous vertebral body ; li. intervertebral ligament ; 

 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 



