20 PBOF. G. E. HOWES AND MB. H. H. SWINNEBTON ON THE 



of the ends of the vertebrae, which during the earlier stages were structurally uniform 

 with the cells adjacent, have at this stage become large and rounded, whereby their 

 matrix is less conspicuous. Once again there are no structures recognizable which 

 merit the term " intervertebral disks," nor is there any discontinuity of parts. 



At this stage a still further numerical reduction of the primary intercentra goes on, 

 their disappearance involving segments 5 to 9 and segment 30 (cf. table, column T, 

 p. 27), leaving only four present in the prsesacral region and traces of a fifth. The 

 third, and probably the second and first, at the same time commence to ossify, which 

 they do endogenously. For certain of the segments which have lost all traces of their 

 primary intercentra, there now takes place the formation of an entirely new set of parts, 

 remarkable no less for their time of appearance than for their detailed structure. These, 

 which arise in relation to segments 9 to 13, appear wholly beneath, though in close 

 apposition medially with, the interarticular masses (.#., PI. 1. figs. 13, 14), but with 

 them they have at first no sort of connection. Laterally quite free of them, they lie 

 between them and a tract of coarse fibrous tissue which (f.t.) besets the ventral face 

 of the vertebral column. Each of these secondary intercentra stains deep yellow, as 

 does ordinary bone. The most remarkable character of them, however, is the presence 

 of a very dark and deeply-staining central portion (cf. fig. 14), which is highly con- 

 spicuous under a high power of the microscope, and into which there pass convergently 

 coarse fibres arising from the underlying tissue, which by their refractability are easy of 

 observation and impart to the bodies a highly distinctive character. These secondary 

 intercentra are from the first median and transverse, and in this they differ in toto 

 from the primary, which arise in pairs. 



. Adult Vertebral Axis. On the anatomy of this we have nothing that is general to 

 add. Develop mentally, the most noteworthy advances, apart from the mere com- 

 pletion of the vertebral bodies, notochordal plates, and chorda, concern the inter- 

 articular tracts (which have become more distinctly fibrous) and the intercentra 

 (which now are finally present along the whole length of the vertebral column). The 

 tail-vertebrse and their special peculiarities will be considered later. 



Goette has recently briefly described l the detailed structure of the fully-formed 

 vertebra of Sphenodon, but that imperfectly, and judging by his figures from apparently 

 none too well-preserved material. He omits mention of the chordal plate, and describes 

 the chorda remnant as passing continuously through the vertebra?. He further remarks 

 upon the peculiar constitution of the osseous tissue of the vertebra, which he regards 

 as " no true bone," but rather to be compared in part to a poorly-celled calcified 

 cartilage, with enclosed medullary nests, and of an exceptional type for the living 

 Amniota. 



We figure longitudinal sections from the sternal region of the adult backbone, 

 stained with haematoxylin (PI. I. tig. 15). The intervertebral tissues are now strongly 

 1 Goette, A. : Zeitschr. wiss. Zool. Bd. Ixii. 1897, p. 364. 



