66 



ZOOLOGY 



SECT. 



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 



sp.cd 



c.c 



the middle 

 of the brain im- 

 mediately behind 

 a peculiar organ, 

 the pituitary body 



p.cJb 



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

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

 ternal elastic membrane ; 7t. ?. haemal 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 



migra 



sp. cd. 



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

 . 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 skeletogenous layer (Fig. 716, sk. L), some of the cells of 



