VERTEBRA 19 



there may be two on a side, a diapophysis on the neurapophysis and 

 a parapophysis in the centrum. Neural and haemal arches of succes- 

 sive vertebrae are connected by dorsal and ventral longitudinal liga- 

 ments (sometimes two dorsal) which run on the chordal side of the 

 spinous processes. 



A vertebra is complex in origin. Reference must be had to special 

 papers for details; many points are uncertain, but an outline of the 

 development is necessary to understand structures and homologies 

 in many groups. 



Vertebras are formed around most of the notochord which extends 

 from the posterior half of the skull to the tip of the tail, just beneath 

 the spinal cord. The notochordal cells, at first distributed through 

 the chorda, rise to its surface where they form an epithelial-like 

 (epitheliomorph) layer (fig. 16) which, with its external limiting 



Fig. 16. — Formation of notochordal sheath in Callorhynchus (Schauinsland, '05). 

 c, cartilage of sclerotome; e, epitheliomorph layer of n, notochord; ei, elastica interna; 

 m, cartilage cells invading s, notochordal sheath; sc, cartilage cells in sheath. 



membrane, is called the elastica interna. The epithehomorph cells 

 secrete a non-cellular envelope, the notochordal sheath, the external 

 layer of which, the elastica externa, differs from the rest. Neither 

 chorda or sheath show metamerism, except as later they are influ- 

 enced by structures developed around them. 



The myotomes (p. 2) lie laterally on either side, each with its 

 myoccele, bounded by somatic and splanchnic walls. The dorsal 

 part of the splanchnic wall (fig. 2) forms the voluntary muscles of the 

 body, which, in lower Vertebrates, largely retain their metamerism 

 through life. The more ventral part of this wall (in lower Verte- 

 brates part of the mesothelium ventral to the myotomes may partici- 

 pate) forms mesenchyme, a part of which develops into the vertebra; 

 hence these mesenchymatous bodies are called sclerotomes. The 

 successive sclerotomes are limited by vertical blood vessels, and each 

 sclerotome is at first divided vertically by a gap into cranial and 



