XIII 



PHYLUM CHORDATA 



trp.cd, 



n.c 



c.c 



(Fig. 760, A, ptij. b.), which will be referred to again in treating of 

 the digestive organs and of the nervous system. The extension 

 of the nervous 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 verte- 

 bral column. The 

 cells of mesoderm 

 surrounding the 

 notochord become 

 concentrated 

 around the sheath 

 and give rise to 

 the skeletogenous 

 layer (Fig. 761, 

 sk.l.), some of the 

 cells of which 

 (sk. c.) may migrate 

 through the elastic 

 membrane into the nc j t c 



sheath itself. In 

 this way the noto- 

 chord becomes sur- 

 rounded by a cel- 

 lular investment 

 which soon takes 

 on the structure 

 of cartilage, and 

 may be called the 

 perichordal tube 

 (Fig. 761, p.c.t. t and 

 Fig. 762, 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-spirial cavity 

 and connected below with the perichordal tube ; and to 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 haemal 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 backwards, beginning a short distance 



p.c.l 



Fie. 7til. Semi-diagrammatic transverse section of the vertebral 

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

 ternal elastic membrane ; h. r. 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 

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

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



