OCT. 1895. VERTEBRAL COLUMN OF AMIA HAY 37 



aortal support (Fig. 12). The latter structure is, of course, even 

 in adult life, not covered with bone at its distal end. Whether the 

 bone found on the two sides of this originates on one side and then ex- 

 tends around it to the other, or whether there are two distinct centers, 

 I am unable now to say. As already remarked, it is found that along 

 the posterior dorsal region the crust of bone is developed a little 

 sooner than the cartilaginous aortal support, so that the latter never 

 lias a direct connection with the main portion of the cartilage of the 

 lower arch. The hindermost of the supports are rather tardy in 

 making their appearance. As already stated, all the intercalated car- 

 tilages, both of the dorsal and of the caudal regions, become ossified, 

 ach from the two centers, which soon unite into one. The upper 

 half-arches of the dorsal and anterior tail regions are incrusted each 

 with a sheath of bone, which arises at an earlier period than do the 

 ossifications closer to the notochord. This sheath covers each half- 

 arch except at the proximal and distal ends. The upper half-arches 

 of the middle tail region and all of the lower half-arches have ossifica- 

 tions at their bases, next to the notochord, each arising from two cen- 

 ters, and have also a sheath of bone covering the greater extent of 

 their length. This sheath, as in the case of the dorsal half-arches, 

 leaves the distal extremity of the cartilage free, and proximally comes 

 down close to the basal ossification, without, however, ever uniting 

 with it at any time of life. We may therefore say that each half- 

 arch has two ossifications, that contiguous to the notochord and the 

 distal sheath, except the upper half-arches of the dorsal and anterior 

 tail regions. These have only the distal sheath, and lack the basal 

 ossification. My judgment is that the latter has been suppressed by 

 the ossification arising from the neighboring intercalated cartilage. 

 The cartilage found in each half-aich is unbroken from the notochord 

 to the tip, except those of the dorsal and anterior tail regions. 



We are now to learn how the vertebral bodies of the different 

 regions are formed. In the trunk and anterior tail regions each cen- 

 trum is produced by the fusion of the bones which have arisen from the 

 upper intercalated cartilages with those at the bases of the correspond- 

 ing lower arches. The upper arch has nothing whatever to do with the 

 formation of the centrum. Its sheath simply rests on the upper sur- 

 face of the centrum, and is separated from it by a permanent suture. 

 A parapophysis of less or greater length forms on each side, a part 

 of each vertebral centrum, being originally a portion of the lower 

 arch. In the anterior tail region there is no parapophysis, the ossifi- 

 cation not extending beyond the surface of the vertebra. 



