14 



PEOF. G. B. HOWES AND ME. H. H. SW1NNERTON 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 (PI. I. figs. 7 & 8, c.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 



Fig. 2. Fig. 3. 



vy 



n.a 



n.c.s 



n,a 





nc.s 



Transverse sections through the poststernal region of the trunk of SpUenodon. 2. Intervertebral ; 3. Vertebral, 



drawn for comparison. Camera lucida X 70. 



c.v. Cartilaginous vertebra ; i.p., primitive paired intercentrum ; my,, myelon ; n.a., neural arch ; nc.e., 

 notochordal epithelium: nc.s., chordal sheath; r.t., trunk rib; s.s. skeletogenous sheath; t.a., inter- 

 articular tissue (intervertebral portion of skeletogeuous 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 

 regarding the centrum as of paired origin a view which lends support to Gadow's 

 conclusion that it represents the fused " interventralia " of the lower vertebrate forms. 



As is well-known, the chevrons and praesternal or cervical intercentra present in 

 most Lacertilia may, by shifting, come into secondary relationship either with the 

 vertebrae in front of them l (by which they may even be carried down on autogenous 

 hypapophyses, as in the neck of the Mosasaur Tylosaurus 2 ), or with those behind 



1 Ex. Anguidap, Varanidae, Helodermatidse ; and, with co-ossification, Mosasaurus, some Varanida?, and 

 Pseudopus. (Of. Boulenger, P. Z. 8. 1891, p. 113, and Gadow, Phil. Trans, vol. 187 B. 1896, p. 30). 



2 Cf. Osborn, H. F. : Mem. Americ. Mus. Nat. Hist. vol. i. 1899, p. 171 ; and also Americ. Nat. vol. xxxiv. 

 1900, p. 1. 



