intersclerotomic (inter— referring to between) and tlie intra- 

 sclerotomic (intra— referring to within) fissures disappear, 

 but the position of the former is marked by the segmental 

 blood vessels, while the ventral ramus of the segmental nerve 

 runs down through the caudal sclerotomite. The notochord 

 remains small in the mammal and never has a distinct 

 sheath. 



Mesenchyme of the sclerotomes eventually encloses the 

 notochord and forms a tube of irregular thickness around 

 it (Figure 6-3). An indistinct perichordal tube, made up of 

 fusiform cells oriented so that they encircle the notochord, 

 is formed within the sclerotomal tissue; elsewhere the irreg- 

 ular mesenchyme cells persist. An intervertebral disc is now 

 apparent which is much denser than the mesenchyme in 

 front of or behind it. This condensation extends laterally 

 along the myotomic septum as the blastematic rudiment of 

 the rib. Exactly what this transverse condensation represents 

 is hard to say, but it is assumed to be the condensed caudal 

 sclerotomite plus a little of the cranial sclerotomite in front 

 of it. The light band, the primary centrum, plus the poste- 

 rior part of the dark band, gives rise to the blastema of the 

 centrum. 



Chondrification appears first in the middle of the primary 

 centrum and encloses the notochord as a vertebral ring. 

 From this center, chondrification spreads throughout the 

 entire centrum. At the same time, the neural arches chon- 

 drify separately as do the proximal ends of the ribs. The 

 neural arches and ribs were primarily associated with the 

 dense intervertebral disc but now attach to the more ante- 

 rior part of the primary centrum (Figure 6-4). 



The neural arches, are joined by a blastema to either side 

 of the neural canal, which when chondrified forms the post- 

 and prezygapophyses of adjacent vertebrae. 



With formation of the completed cartilaginous vertebra, 

 the dark intervertebral band is narrowed and now lies on 

 the plane between the ventral nerve rami of the segment 

 and somewhat anterior to the original position of the intra- 



sclerotomic fissure. As this dark band narrows, the fibrous 

 intervertebral disc is formed from it. 



Endochondral ossification centers appear '\n the arches, 

 then in the centrum and ribs. The centers grow outward 

 to form the definitive, or fully developed, structure. The 

 epiphyses of the vertebral body appear late, following the 

 elimination of the notochord within the centrum. Their 

 appearance is followed by fusion of the arches dorsally and 

 then with the centrum. The centrum is pierced dorsally, 

 and just medial to the roots of the neural arches, by bilat- 

 eral, large nutritive foramina. 



General observations .-Xmong mammals the vertebral 

 column shows a wide range of variation but it is always 

 divisible into segments: cervical, thoracic, etc. In its ossifica- 

 tion the mammalian centrum is distinctive in that the bone 

 arises endochondrally and in that there are epiphyses on the 

 ends of the centrum. Of interest is the fusion of the centra of 

 the first and second vertebrae to form the body of the axis; 

 this is accomplished without the intercession of epiphyses. 

 The posterior end of the definitive axis centrum has an 

 epiphysis. Epiphyses are present only in the caudal series of 

 the monotremes, and are lacking throughout the column in 

 Sirenia. 



Between the vertebrae are intervertebral discs. The inter- 

 vertebral disc may be little more than connective tissue 

 binding succeeding vertebral bodies, or it may be a distinct 

 pad of fibrous connective tissue, as in man. In this connec- 

 tive tissue, there is sometimes a nucleus pulposus, a mass of 

 vacuolated cells representing the remains of the notochord. 



Reptiles and bird 



A variety of reptiles, alligator, lizard, and .Sphenodon, will 

 be examined along with the bird. The bird, as a modified 

 archosaur, is in most respects a reptile and as such is de- 

 scribed here. 



rib rudiment 



intersegmental vessel 

 sclerotomic fissure 



,*P 



nal ne 



sclerotomic fissure 



intervertebral disc 



primary centrum 



i -■'■-;■..:■■■: ..,. '.i^'4- myoseptum 





vertebra - 





rib rudiment 



spinal nerve 



intervertebral disc 



vertebral artery 



perichordal tube 



notochord 



B 



Figure 6-3. Blastematic stages of vertebrae in the human. (After Sensenig, 1949) 



144 • THE VERTEBRATE BODY SKELETON 



