302 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. 



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

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

 the perichordal layer. Eventually chondrification takes place and the 

 vertebral body, derived partly from perichordal and partly from 

 sclerotouie 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 perichordal 

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



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 



I .v. '"& 



N f 



p.v.h 



&. 



FlG. 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, rnyotomi- ; A, notochord ; j'.r./i, primary vi-rd-hral body ; N, superficial portion of centrum 

 .'itid- prrirhurdal luyi-r ; f.'.i, spinal ^an-lion ; *</, sclcrotonn- ; -, blond-xrss.-I. hi (.m|rinK 

 the Hegmentui relations of A and H tin- int.-rs.-ni'-ntal blood-vessels (<') form usHul landmarks. 



vertebra in Sphenodon and in the Lacertilia. In the ordinary 

 Li/.ards this appears to arise as a ring-shaped ingrowth of cartilage 

 \\hieh (-'districts the uotochord, pushing the primary sheath in front 

 of it (fiadow, 1897) while in Sphenodon and also in the (Jeekos the 

 cartilage makes its appearance internal to the notochordal sheath 

 (Howes and Swinnerton, I'.iOl;. It may le suspected tliat in the 

 latler case iliinii.Lji.ml cartilage cells ha\c made their \\ay through 

 the notOfilmnl.-Ll sheath though this ha- not so far lu-eii demons! rated. 



1,'r Tlicj ribs are long cartila-mous projections from the 

 vertebrae \\-hieh run outwardi and \e t iiiall\ m t IMJ substance ol the 

 myoKcpta and serve t<> support and strengthen the wall of the 

 IpWcnnOOOela As Groette . ISV.S, 1879) first showed, there are 



included under I he name "ribs" I \\ morpholo-ieally ililfereiil 



