DEVELOPMENT OF THE SKELETON OF THE TUATAEA. 



21 



fibrous, and on the ventral side they, together with the subvertebral fibrous tissue {f.t.), 

 have attained to full differentiation. Their cells are arranged in parallel series, with 

 an accompanying loss of individuality, the tissues having the appearance of being 

 permeated by widely-separated parallel rows of small and elongated nuclei. While 

 peripherally this parallel arrangement becomes more conspicuous, in the deeper layers 

 the fibres become stronger, their attachments to the bony vertebrae appearing more 

 marked, as the result of their greater avidity for the stain. Owing to the increase in 

 thickness of the bony shell of the vertebral body (still largely composed throughout 

 its inner moiety of calcified cartilage), the chordal plates (PI. I. fig. 15 and text-fig. 4, 

 n.p.) are comparatively insignificant as compared with the earlier stages. They are, 

 however, present, and, owing to their completely septate nature, the chorda is now 

 broken up into a series of elongated and recurrent interseptal segments, each of which 

 is seen to be bounded not by the chordal epithelium hitherto recognizable, but by a 

 deeply-staining and structureless cuticle, which passes into the faces of the plates, the 

 superficial portions of which stain correspondingly with it. 



Fio;. 4. 



Median longitudinal section of the adult vertebral column (caudal region) of Splienodon, to show the tunica 



chordae (i.e.) and its relationships to the chordal sheath and plate. Camera lucida, x 70. 



i.sg., interseptal segment of chorda ; n.cs., chordal sheath; n.p., chordal plate ; o.v., osseous vertebra; 



t.c. tunica ohordse. 



This cuticle, which we propose to term the tunica chordce (text-fig. 4, t.c), would 

 appear to represent the chordal epithelium ; but that it underlies and is independent 

 of the chordal sheath {nc.s.) is proved by the fact that this stains lightly and can 



