v VERTEBRAL CENTRA 301 



The notochord becomes more or less constricted by the ingrowth 

 of the intervertebral or joint cartilage which pushes the sheath in 

 front of it. Besides this eonstriction of the whole notochord 

 with its sheath the substance of the notochord becomes eventually, 

 sometimes at a relatively late stage of development, interrupted by 

 the development of -m braver tebral cartilage which may form a com- 

 plete cartilaginous partition across the notochord at about the middle 

 of each vertebra (Fig. 151, B, nc). The origin of this cartilage is 

 disputed. Some (Lwoff, Zykoff, Gadow) derive it from immigrant 

 cartilage cells which have penetrated through the notochordal sheath 

 from outside, while others (Gegenbaur, Field, Ebner, Klaatsch, 

 Schauinsland) believe it to originate by the metamorphosis of actual 

 notochordal cells, probably cells of the notochordal epithelium. In 

 spite of a possibly greater volume of evidence supporting the latter 

 view it is difficult to avoid the impression that the former has in its 

 favour the balance of a priori probability. 



The lteptiles are commonly regarded as the least specialized of the 

 three subdivisions of the A inn iota and it may therefore be con- 

 venient to let them form the basis of our description. Schauinsland's 

 work may be referred to for more minute detail. 



The sclerotome tissue grows actively and comes to be specially 

 concentrated immediately round the notochord to form the peri- 

 chordal layer. This layer is at first — in accordance with its origin 

 from the sclerotomes — segmented (Fig. 152, A, scl) but the original 

 segmentation soon disappears so that it forms a perfectly continuous 

 investment to the notochord. A secondary segmentation now 

 becomes visible in as much as the perichordal layer is decidedly 

 thicker in a position corresponding to the middle of each original 

 segment. These thickenings mark it off into a series of reel-shaped 

 pieces each of which is a primary vertebral body (Fig. 152, B and C, 

 p.v.b). It will be understood that the hinder half of each primary 

 vertebral body is derived from the front half of a sclerotome while 

 the front half of the same primary vertebral body is derived from 

 the posterior half of the next sclerotome in a headward direction. 



In other words each primary vertebral body is formed from the 

 adjoining halves of two original segments, and as a result of this the 

 primary vertebral bodies necessarily alternate in position with the 

 myotomes, each myotome running from about the level of the middle 

 of one primary vertebral body to a level about the middle of the next 

 in the series (Fig. 152, B). 



The portions of the sclerotomes lying outside the perichordal 

 layer undergo fusion also. This outer part of the sclerotome bulges out 

 between the myotomes while it extends dorsalwards so as to arch 

 over the spinal cord. It is in the wall of the tunnel so formed that 

 the neural arch-elements make their appearance while the sclerotome 

 tissue ventral to them takes part in the formation of the body of the 

 definitive vertebra. The superficial part of the vertebral body arising 

 in this way from sclerotome tissue outside the perichordal layer 



