138 HAY. [You I. 



two of bone. The latter develop so cunningly that they unite 

 not the same elements that were united by the cartilage, but the 

 basiventral with the basidorsal of the next myotome behind, 

 and the interventral with the interdorsal of its own myotome. 



The scheme of development worked out by the authors is a 

 complex one, and difficult to comprehend ; but after having 

 given, I believe, sufficient attention both to it and to a com- 

 parison of their statements with actual sections, I feel prepared 

 to make the following remarks. 



The cartilaginous arcualia do not rest on any layer of con- 

 nective-tissue bone. They come into direct contact with the 

 elastica externa and remain in contact with it. 



There are from the arcualia no downward and upward out- 

 growths of cartilage, not even of fibro-cartilage, which meet and 

 fuse to form rings around the notochord outside of any layer of 

 connective-tissue bone. The earliest bone formed starts appa- 

 rently as a ring around the base of each arcuale where it rests 

 on the elastica. From its place of origin it spreads in all 

 directions. In any section it will be seen rising in one direc- 

 tion against the surface of the cartilage and spreading in other 

 directions over the notochordal sheath. Any outgrowth of carti- 

 lage from the arcuale such as described would have to break 

 through this sheath of bone. So far as I can discover, all the 

 bone of the centrum is derived from this earliest stratum by in- 

 crease in thickness and later by the sending out of anastomos- 

 ing plates into the surrounding connective tissue. After care- 

 ful comparison of the figures of the paper of Gadow and Abbott 

 with sections of specimens of various sizes up to 125 mm., I 

 have come to the conclusion that those alleged outgrowths are 

 little, if anything, more than dense masses of connective tissue 

 which occur in the region of the articular ends of the vertebras. 

 I can discover no cartilage cells outside of the early formed 

 layer of bone. In a specimen 125 mm. long every portion of 

 the original cartilage is yet distinguishable, and I find no trace 

 of such outgrowths or even room for them. We are told that 

 it is the bone belts arising in these outgrowths which bind to- 

 gether the various dorsal and ventral pieces and which constrict 



