int PHYLUM CHORDATA 71 



word, forms one of the most complex portions of the body, and 

 presents an immense range of variation in the different classes and 

 orders. As in Amphioxus, the axis of the entire skeletal system 

 is formed by the notochord (Fig. 769, nch.}, an elastic rod made of 

 peculiar vacuolated cells (Fig. 770, 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, in the 

 Craniata, 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 ccelome below : 

 it is usually developed, as in the lower Chordata, from a median 

 longitudinal outgrowth of the dorsal wall of the gut. Posteriorly 

 it extends 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 

 immediately . 



behind a pecu- ** / 



liar organ, the 

 pituitary body 

 (Fig. 769, A, 

 pty. b.}, which 

 will be referred 

 to again in 

 treating of the 

 digestive organs 

 and of the ner- 

 vous System. -p K} 7 7 j_ Diagram illustrating the segmentation of the vertebra 

 The extension column, c. n. t. perichordal tube; h. r. liicmal ridge ; h. t. haema 



. . tube ; i. c. /. intervertebral > foramen ; n. t. neural tube ; nch. 



Of the nerVOUS notochord. The dotted lines indicate the segmentation into 



system in front 



of the notochord 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 cells of mesoderm surrounding the 

 notochord become concentrated around the sheath and give rise 

 to the skeletogenous layer (Fig. 770, sk. I.), some of the cells of which 

 (sk. c.) may migrate through the elastic membrane into the sheath 

 itself. In this way the notochord becomes surrounded by a cellular 

 investment which soon takes on the structure of cartilage, and may 

 be called the perichordal tube (Fig. 770. p.c.t., and Fig. 771, c.n.t.}. 

 The skeletogenous layer also grows upwards, and gives rise to an 

 inverted tunnel of cartilage, the neural tube (n.c., n.t.}, enclosing 

 the cerebro-spinal cavity and connected below with the perichordal 

 tube ; and to paired hcemal ridges (h.r.) of cartilage standing out 



F 2 



