4 STRUCTURE OF THE VERTEBRATES 



chordate as a fish with persistent notochord and gills, although 

 the notochord is crowded out of existence during late foetal life 

 or infancy, and the gill slits are present for only a short period. 



Notochord. The notochord is an elastic rod of tissue acting 

 as a supporting structure of the body. It is the only dorsal sup- 

 port of the primitive chordates, but in the vertebrates it becomes 

 surrounded by the cartilage or bone which forms the vertebrae, 

 and it is completely lost in the adults of the higher groups. 



The notochord arises from endodermal cells which lose their 

 nuclei, and it then becomes an apparently homogeneous, non- 

 cellular mass of tissue surrounded by sheaths of connective 

 tissue. Its position is dorsal to the primitive gut and ventral 

 to the nerve cord. 



Nerve Cord. The vertebrate brain and nerve cord, which 

 together form the central nervous system, are hollow and dorsal 

 in position. In both characteristics they differ from all other 

 animals, the invertebrates having a solid nerve cord, ventral in 

 position. 



The chordate nervous system arises from ectodermal tissue 

 as a groove along the dorsal surface of the developing embryo. 

 The significance of this dorsal position will be apparent to the 

 student if he will consider the great development of the brain 

 in the higher vertebrates, and its position in relation to the 

 digestive tract. 



Pharyngeal Gill Slits. All chordate embryos develop gill slits 

 in the pharynx, that region of the digestive tract between the 

 mouth cavity and the esophagus. These gill slits are retained 

 by the adults in all the primitive chordates; and, aniong the 

 vertebrates, in the fish and some of the salamanders. In frogs 

 they are found in the tadpoles but are lost in the adults. In 

 higher vertebrates the gill slits disappear in the embryo, except 

 for the first slit which remains as the cavity of the middle ear. 



The slits arise in the embryo as paired, endodermal outpocket- 

 ings from the pharynx, extending laterally toward the sides of 

 the animal. As each pouch approaches the ectoderm the latter 

 forms a depression at the point of contact. The membrane 

 formed at this point by the two layers breaks, forming a tube 

 connecting the pharynx with the outside. 



