THE ENDOSKELETON: VERTEBRAL COLUMN AND RIBS 



59 



Fig. 10, p. 43). The sclerotome migrates to the median region of the embryo and surrounds the 

 notochord and neural tube. It then gives rise to cartilage, which is laid down around the 

 notochord. The vertebrae may remain permanently in this cartilaginous state, as in elasmo- 

 branchs, being stiffened by calcification, or they later transform into bone, as in the majority 

 of vertebrates. The student should understand clearly that the vertebral column does not 

 come from the notochord but is formed around it. The notochord is inclosed inside the 

 vertebral column, and is readily located in this position in the more primitive fishes; but in 

 most vertebrates it is gradually squeezed down and disappears except for remnants between 

 the vertebrae. 



A sclerotome does not, as might be supposed, produce a vertebra as one piece. The 

 sclerotomes are segmentally arranged, there being a sclerotome medial to each myotome. It 

 has already been stated, however, that the vertebrae alternate with the myotomes and arise 

 at the intersection of myosepta with the median skeletogenous region. This result is achieved 

 as follows. Each sclerotome soon becomes divided by a vertical split into an anterior or 

 cranial half, posterior to the plane of the preceding myoseptum, and a posterior or caudal half, 

 anterior to the plane of the succeeding myoseptum. A vertebra arises from the fusion of the 



ectoderm 



notochord 



anterior half 

 of sclerotome 



posterior half 

 of sclerotome 



myotome 



intersegmenta] _ 

 artery 



otochord 



posterior half 

 of sclerotome 



anterior half 

 of sclerotome 



^ ^> vertebra 



intersegmental 

 artery 



FIG. 16. Two stages in the development of the vertebrae from the sclerotomes, only one side of 

 the body being shown. A, division of the sclerotome into anterior and posterior halves. B, union of 

 the posterior half of one sclerotome with the anterior half of the succeeding sclerotome to form a 

 vertebra. (From Prentiss and Arey's Textbook of Embryology, courtesy of the \V. B. Saunders 

 Company.) 



posterior half of one sclerotome with the anterior half of the succeeding sclerotome. The 

 anterior half of one side of the vertebra comes from the posterior half of a sclerotome and 

 the posterior half of one side of the vertebra from the anterior half of the sclerotome. (It 

 should be understood that the sclerotomes of the two sides of the body axis co-operate in the 

 formation of each vertebra.) It thus happens that the center of the vertebra intersects a 

 myoseptum and that the vertebrae alternate with the myotomes (see Fig. 16). 



2. The arcualia. In the formation of a vertebra definite paired cartilages known as 

 arcualia arise in the sclerotome. Theoretically there are four pairs of arcualia to each vertebra . 

 The posterior halves of the two sclerotomes of each segment, which form the anterior half of 

 a vertebra, produce two pairs of arcualia a dorsal pair above the notochord, called the 

 basidorsals, and a ventral pair below the notochord, called the basivenlrals. Similarly 

 the anterior halves of the two sclerotomes of each segment, which together form the 

 posterior half of a vertebra, produce a dorsal pair of arcualia, the interdorsals, and 

 a ventral pair, the interventrals (see Fig. 17). There may be other elements in addition 

 above and below these, such as the supradorsals and infraventrals, but these are not 

 of general occurrence. The members of the chief pairs of arcualia tend to fuse together 

 medially producing arches or pieces which straddle the notochord above and below. Thus 



