xiii PHYLUM CHORDATA 67 



which (sk. c.) may migrate 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. 716, p.c.t., and Fig. 717, c.n.t). The 

 skeletogenous layer also grows upwards and outwards, 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 paired hcemal ridges (h.r.) of 

 cartilage standing out from the sides of the perichordal tube into 

 the muscles : in the region of the tail these unite below to enclose 

 the hcemal canal (h.t.) already referred to. Actually, however, the 

 vertebral column thus constituted is from the first more or less 

 broken up into segments, and in the higher forms is replaced by a 

 chain of bones called vertebra; which follow one another from before 



i.v.f 



tt.C I *J 



-p - 



f JL Mil 



FIG. 717. Diagram illustrating the segmentation of the vertebral column, c. n. t. perichordal 

 tube; h. r. haemal ridge; h. t. haemal tube; i. r. f. inter-vertebral foramen; n. t. neural 

 tube ; nch. notochord. The dotted lines indicate the segmentation into vertebras. 



backwards, beginning a short distance behind the anterior end of 

 the notochord and extending to the extremity of the tail. 



A vertebra consists essentially of the following parts : (1) a 

 centrum or body (Fig. 715, C, en.) lying below the spinal canal in the 

 position formerly occupied by the notochord and perichordal tube, 

 and arising either in the skeletogenous layer proper, or in the 

 notochordal sheath after its invasion by skeletogenous cells ; (2) a 

 neural arch (n. a.) which springs from the dorsal surface of the 

 centrum and encircles the spinal canal, representing a segment 

 of the neural tube; and (3)" a pair of transverse processes (t.p.) 

 which extend outwards from the centrum among the muscles 

 and represent segments of the haemal ridges : to them are often 

 attached ribs which extend downwards in the body-wall, some- 

 times between the dorsal and ventral muscles (r 1 ), sometimes 

 immediately external to the peritoneum (r). In the anterior part 

 of the ventral body- wall a cartilaginous or bony sternum or breast- 

 bone may be developed : in the Amphibia it is an independent 

 structure ; in the higher classes it is formed by the fusion of some 



F 2 



