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rirYLUM CMORDATA 



73 



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(Fig. 7G0, A, ptij.h.), whioh will he ivfiM-ivd 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 5ne 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 antci-ior end of the nerve-tube. 



In the majority of Craniata the notochord is a purely embi'vonic 

 structure, and all l)ut the anterior end of it is replaced in the 

 adult by the vcrtc- 

 hral column. The 

 cells of mesoderm 

 surrounding the 

 notochord become 

 concentrated 

 around the sheath 

 and give rise to 

 the skeldogcnous 

 layer (Fig. 761, 

 sl:l.), some of the 

 cells of which 

 (sk. e.) may migrate 

 through the elastic 

 membrane into the 

 sheath itself. In 

 this way the noto- 

 chord becomes siir^ 

 rounded by a cel- 

 lular investment 

 which soon takes 

 on the structure 

 of cartilage, and 

 may be called the 

 per ichor dal tube 

 (Fig. 761,29.c.^.,and 

 Fig. 762, c.n.t.). 

 The skeletogenous 

 layer also grows 

 upwards, and gives 

 the neural tube {nj 



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Fki. 7(')1. — Scmi-dingraiiimatic traiisvorse section of the vertebral 

 column of a craniate embryo, c c. ccntial canal ; d. m. ex- 

 ternal elastic membrane ; /(. r. lin'iiial ridges : n. c. neural tube ; 

 ncli. notochord ; itrli. r. notuchordal colls ; /(. r. t. pcrichordal 

 tube; sh. nrh. sheath of notochord ; si: c. skeletogenous cells 

 migrating into notochordal sheath ; sk: /.skeletogenous layer; 

 sp. cd. spinal cord. (Modified from Balfourand Gadow.) 



rise to an inverted tunnel of cartilage, 

 n.t.), enclosing the cerebro-spinal cavity 

 and connected below with the perichordal tube ; and to paired 

 hannal 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 lunnal 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 



