66 



ZOOLOGY 



SECT. 



sp.cd 



is formed by the notochord (Fig. 715, nch.), an elastic rod made of 

 peculiar vacuolated cells (Fig. 716, nch.), resembling the pith of 

 plants, and covered by a laminated sheath (sh. nch.), with an 

 external elastic membrane (el. m.) around it. The whole sheath is 

 a cuticular product of the superficial notochordal cells (nch. c.). 

 i.e., is developed as a secretion from their outer or free surfaces. 

 The notochord lies in the middle line of the dorsal body- wall 

 between the cerebro-spinal cavity above and the coelome below : 



it is usually de- 

 veloped, as in the 

 lower Chordata, 

 from a median 

 longitudinal out- 

 growth of the dor- 

 sal wall of the gut. 

 Posteriorly it ex- 

 tends to the end 

 of the tail, but in 

 front it always 

 stops short of the 

 anterior end of 

 the head, ending 

 near the middle 

 of the brain im- 

 mediately behind 

 a peculiar organ, 

 the pituitary l>o<lt/ 



c.c 



p.c.l 



FIG. 716. Serni-diagrammatic transverse section of the vertebral 

 column of a craniate embryo ; c. c. central canal ; el. m. ex- 

 ternal elastic membrane ; h. r. hamial ridges ; n. c. neural tube ; 

 nch. notochord ; nch. c. notochordal cells ; p. c. t. perichordal 

 tube ; sh. nch. sheath of notochord ; sk. c. skeletogenous cells 

 migrating into notochordal sheath ; sk. I. skeletogenous layer ; 

 sp. cd. spinal cord. (Modified from Balfour and Gadow.) 



which will be re- 

 ferred to again in 

 treating of the 

 digestive organs 

 and of the nervous 

 system. The ex- 

 tension of the 

 nervous system in 

 front of the noto- 

 chord is one of the most striking differences between the Craniata 

 and Amphioxus, in which it will be remembered the notochord is 

 prolonged to a considerable distance beyond the anterior end of 

 the nerve-tube. 



In the majority of Craniata the notochord is a purely embryonic 

 structure, and all but the anterior end of it is replaced in the adult 

 by the vertebral column, the structure to which the entire vertebrate 

 sub-phylum owes its name. The cells of mesoderm surrounding 

 the notochord become concentrated around the sheath and give 

 rise to the skeletoymous layer (Fig. 716, sk. /.), some of the cells of 



