14 PROF. G. B. HOWES AND MR. H. H. SWINNERTON ON THE 
skeletogenous sheath, in both dorsal and ventral middle lines the latter (s.s.) has 
remained unchanged. 
Comparison with the foregoing of a lateral longitudinal section at a slightly later 
stage, shows (Pl. I. figs. 7 & 8,¢.v.) that these lateral chondrites encroach upon the 
skeletogenous sheath from without inwards, rapidly involving its deeper layers, until its 
innermost limit is reached. ‘They also effect the replacement of the afore-mentioned 
median portions, but by union within the deeper layers and subsequent extension in 
the opposite direction—/. e. from within outwards, as is seen at c.v. in Pl. I. fig. 6, 
which is that of a median longitudinal section at this period. 
The above described processes continue until chondrification of the vertebral portions of 
the skeletogenous Sheath is complete, and there thus results a metameric segmentation 
Transverse sections through the poststernal region of the trunk of Sphenodon. 2. Intervertebral; 3. Vertebral, 
drawn for comparison. Camera lucida x 70. 
cv. Cartilaginous vertebra; i.p., primitive paired intercentrum; my., myelon; n.a., neural arch; xe.e., 
notochordal epithelium: ne.s., chordal sheath; ».¢., trunk rib; s.s. skeletogenous sheath ; ¢.a., inter- 
articular tissue (intervertebral portion of skeletogenous sheath ). 
of the vertebral axis, whereby a series of segments become recognizable, each comprising 
a centrum, an intercentrum, and a pair of neural arches; and the facts justify our 
regardiug the centrum as of paired origin—a view which lends support te Gadow’s 
conclusion that it represents the fused “ interventralia” of the lower vertebrate forms. 
As is well-known, the chevrons and preesternal or cervical intercentra present in 
most Lacertilia may, by shifting, come into secondary relationship either with the 
vertebre in front of them’ (by which they may even be carried down on autogenous 
hypapophyses, as in the neck of the Mosasaur Tylosaurus?), or with those behind 
' Ha. Anguide, Varanidee, Helodermatide ; and, with co-cssification, Mosasaurus, some Varanide, and 
Pseudopus. (Cf. Boulenger, P. Z. 8. 1891, p. 113, and Gadow, Phil. Trans. vol. 187 B. 1896, p. 30). 
* Cf. Osborn, H. F.: Mem. Americ. Mus, Nat. Hist. vol. i. 1899, p. 171; and also Americ. Nat. vol. sxxiy. 
1900, p. 1. 
