SKELETON 5 1 



nost primitive condition arches alone occur, and these may even 



)ssify without any appearance of distinct centra. 



The vertebrae (and the basal part of the skull) are developed 

 around the notochord. This is a cylindrical rod of cells, arising from 

 the entoderm,^ and extending in the middle line of the body, between 

 the central nervous system and the alimentary canal, from the 

 infundibulum (see brain) to the posterior end of the body. In its 

 early stages it shows no signs of metamerism. Soon the cells of which 

 it is composed become vacuolated, and the nuclei and most of the 

 nrotoplasm migrate to the surface of the rod where they appear like 





H 



Fig. 45. — Section of embryo lizard {Lacerta muralis) with twenty -eight somites, 

 the section passing through the tenth somite. To show the proliferation of mesenchyme 

 from the splanchnic layer of the myotome, a, aorta; /, limb-bud; m, part of myotome 

 to form muscle; n, notochord; 0, omphalomesenteric vein; sc, spinal cord; sk, sclerotomic 

 part of mesenchyme. 



an epithelium, which, together with its basal (external) membrane 

 is called the elastica interna or epitheliomorph layer (i&g. 44, e). 



This internal elastic layer now secretes a structureless envelope, 

 the notochordal sheath, which encloses the whole cord, its outer 

 layer, the elastica externa (fig. 44, cs^) being somewhat sharply 

 marked off from the rest. This condition of the sheath persists 

 throughout life in the cyclostomes and a few other forms, but is 

 transitory in all other vertebrates. 



The skeleton-forming (skeletogenous) tissue is derived from the 

 sclerotomes (p. 20). Its history in outline is as follows: Each epime- 

 ral and mesomeral somite buds cells from its medial and splanchnic 

 surface (fig. 45) ; these cells form the sclerotomes, which, since they 

 arise from metameric structures are at first also metameric. But 



• For the entodermal origin of the notochord see p. 13. 



