302 EMBBYOLOGY OF THE LOWEB VEETEBEATES ch. 



(Fig. 152, C, S) is best developed laterally (Sphenodon) though it 

 extends as a thinner layer over both the dorsal and ventral sides of 

 the periohordal layer. Eventually ehondrification takes place and the 

 vertebral body, derived partly from periohordal and partly from 

 sclerotome tissue lying outside and continuous with the neural arch 

 portion, becomes converted into a mass of cartilage in which the 

 only clue to its compound origin is the somewhat flattened shape of 

 the cartilage cells in the inner part derived from the periohordal 

 layer (Fig. 152, C, p.v.l). 



During the development of the vertebral centra the notochord 

 becomes constricted across much as in Urodeles. A complete 

 septum of notochordal cartilage is formed across the middle of each 



scl. -', 



w\- m ^ 



B P- 



<.u.b.' 



;||f-«y. 



^frr-s. 



Fig. 152. — Diagram illustrating the mode of development of the vertebral centra in a 

 Reptile as seen in horizontal sections. (Based mainly on Schauinsland's figures of 

 Sphenodon, 1906.) 



my, myotome ; N f notochord ; p.v.b, primary vertebral body ; S, superficial portion of centrum 

 arising outside perichordal layer ; s.g, spinal ganglion ; scl, sclerotome ; v, blood-vessel. In comparing 

 the segmental relations of A and B the intersegmental blood-vessels (v) form useful landmarks. 



vertebra in Sphenodon and in the Lacertilia. In the ordinary 

 Lizards this appears to arise as a ring-shaped ingrowth of cartilage 

 which constricts the notochord, pushing the primary sheath in front 

 of it (Gadow, 1897) while in Sphenodon and also in the Geckos the 

 cartilage makes its appearance internal to the notochordal sheath 

 (Howes and Swinnerton, 1901). It may be suspected that in the 

 latter case immigrant cartilage cells have made their way through 

 the notochordal sheath though this has not so far been demonstrated. 

 Eibs. — The ribs are long cartilaginous projections from the 

 vertebrae which run outwards and ventrally in the substance of the 

 myosepta and serve to support and strengthen the wall of the 

 splanchnocdele. , As Goette (1878, 1879) first showed, there are 

 included under the name "ribs" two morphologically different 



