32 PROF. G. B. HOWES AND MR. H. H. SWINNERTON ON THE 



On examination of transverse sections through the developing vertebral column 

 of Sphenodon at Stage P, at which the parts of the cartilaginous vertebrae are as yet 

 undifferentiated, the tissue giving rise (PI. I. figs. 3 & 4) to the paired intercentra 

 (i.p. 1 ) which, it will be remembered, are the first, and at this stage the only components 

 of the vertebral column recognizable is seen to be continuous with that differentiating 

 into the ribs (r.cd.). Figs. 3 and 4 represent the first and sixth sections across an 

 individual caudal segment ; and comparison of those which intervene has shown us 

 that even here, when the vertebral body is not yet differentiated, the disposition of the 

 proximal end of the rib is oblique, involving ventrally and anteriorly the intercentrum, 

 dorsally and posteriorly the region of the future centrum and arch. Comparison with 

 the adult cervical rib reveals a close similarity of relationship ; and since with later 

 development the connection with the intercentrum is lost, the primary relationship of 

 the rib is proved to be intervertebral. 



Cope in 1878 first drew attention l to the existence in certain Permian Pelycosauria 

 of a two-headed condition of the rib, with accompanying intercentral articulation of 

 the capitulum ; and Baur, eight years later, in his afore-mentioned paper on the 

 cervical ribs of Sphenodon (prompted by Cope's announcement), sought to emphasize 

 the importance of this. Cope, immediately afterwards, in dealing with the structural 

 resemblances between these creatures, prompted by Hofmann's brilliant generalization 

 afore-mentioned, penned the remarkable paragraph 2 : " The passage of time has 

 seen in the Reptilia generally " a modification " in the mode of attachment of the 

 rib," in which, "in passing from front to back, the intercentral articulation is 

 lost", and finally the head of the rib disappears." The facts we have just described 

 render this well-nigh prophetic ; and it remains to be ascertained how far the sup- 

 position, deduced from the study of longitudinal sections alone, that the ribs of the 

 Amniota, and of the Reptilia in particular, arise in exclusive relationship to the 

 transverse process, centrum, or arch, will stand the test of examination in other 

 planes, and, as deduced from that of developmental stages at which chondrification 

 is more or less advanced, will bear further comparison with the earlier ones of pro- 

 cartilage differentiation. 



Concerning the bodies of the ribs, we have nothing to add to the descriptions 

 of Giinther, Maurer (96), Osawa, and others, except as regarding the so-called 

 " uncinate processes." Giinther in his original description accords the first of these to 

 the last praesternal rib, Osawa to the penultimate, but neither state their posterior 

 limitations. Like those of the Ratite birds, they are apt to be easily lost in maceration ; 

 but in carefully prepared specimens we have been able to trace them back, with 



1 Cope, E. D. : Proc. Amer. Philos. Soc. vol. xvii. 1878, pp. 518 et seq.; and Proc. Amer. Assoc. Adv. Sci. 

 1884, pt. ii. p. 474. 



2 Cope, E. D. : Trans. Amer. Philos. Soc. vol. xvi. 1886, p. 249. 



