14 



PKOF. G. B. HOWES AND MK. H. H. SWINNERTON ON THE 



skcletogenous sheath, in both dorsal and ventral middle lines tlie latter {s.s.) has 

 remained unchant^od. 



Comparison with tlic foregoing of a lateral longitudinal section at a slightly later 

 stage, shows (PI. I. figs. 7 & 8, c.y.) tliat 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 rei)laceraeut of the afore-nientioucd 

 nu.'dian portions, but by union within the deeper layers and subsequent extension in 

 the opposite direction — i. e. from within outwards, as is seen at c.v. in PI. I. fig. G, 

 which is that of a median longitudinal section at tliis period. 



Tlie above described processes continue until chondrification of tlie vertebral portions of 

 the skeletogenous sheath is complete, and there thus results a metameric segmentation 



Fig. 2. 



^a. 







7 



.m. 



w 



Fig. 3. 



■ ■-,'■ it'- i''^"-j.' 

 'iv.V ' ■■'■ 



m^ 







/!/Z 



nee 



-^it^j^". 



Transverse sectious through the poststernal region of the tjuuk of Splie,nodon. 2. Intervertebral ; .3. Vertebral. 



drawn for comparison. Camera lucida x TO. 



c.v. Cartilaginous vertebra ; i.p., primitive paired intercentrum ; nuj., 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 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 

 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 

 vertebrse in front of them ' (by which they may even be carried down on autogenous 

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



' E.r. Anguidff, Varanida;, Helodermntidfe ; and, with fo-os?ification, ilosasunrus, some Yaranidff, and 

 Paeudopus. (Of. Boulenger, P. Z. S. 1891, p. 113, and Gadow, I'hil. Trans, vol. 187 B. 1896, p. 30). 



' Of. Osborn, H. F. : Mem. Americ. i[us. Nat. Hist. vol. i. 1899, p. 171 ; and also Amerio. Nat. vol. xssiv. 

 1900, p. 1. 



