52 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



each sclerotome is also divided by a vertical incision into an anterior 

 or cranial and a posterior or caudal half (fig. 46), a point of great 

 importance in explaining certain vertebral peculiarities. 



The sclerotome halves gradually extend medially to the axial 

 structures and their cells become most abundant in the triangular 

 grooves — two on either side — between the spinal cord and the 

 notochord above, and the notochord and the blood-vessels below. 

 From these tracts the cells extend dorsally, gradually enclosing the 

 spinal cord, while ventrally the blood-vessels are similarly enclosed. 

 With the formation of cartilage these collections of cells develop 

 the neural and haemal arches, and, since there are two sclerotome 

 halves to a somite, there may occur, as in the adult lamprey, two 

 arches to each segment of the body. In the cyclostomes the verte- 



FiG. 46. — Horizontal section through embryo snake (Tropidonotus) at the level of 

 the notochord and the lower part of the myotomes, after Corning. Cells are shown 

 budding from the splanchnic wall of each myotome, and in three places the resulting 

 sclerotomes are plainly divided into anterior (a) and posterior; (/>) sclerotome halves; 

 c, myocoele; iv, intersegmental blood-vessels; n, notochord. The section passes through 

 the middle of the myotomes, cf, fig. 45. 



bral development goes no farther than this, no centra being formed 

 (fig. 82). The vertebral column thus consists in these animals (and 

 the same holds true for many fossil ganoids) of a series of arches rest- 

 ing on an unchondrified notochordal sheath. In the myxinoids the 

 cranial and caudal arches are about equal in size, but in most verte- 

 brates the caudal are the larger, the cranial often being entirely lost 

 in development. 



In most gnathostome vertebrates the centra are formed by ex- 

 tensions from the bases of the neural and haemal arches, but in two 

 different ways. In the one the skeletogenous cells extend from the 

 bases of the arches around on the outside of the elastica externa and 

 later chondrify, the notochordal sheath in this case being unmodified, 

 sometimes throughout life. In the other case cells from the bases 



