THE NOTOCHORD. 255 



In the longitudinal section of the type of the Vertebrate, 

 a thin hut firm rod, of cylindrical form, and pointed at the 

 posterior and anterior ends (Fig. 52, x), is seen in the middle 

 of the body. This passes through the whole length of the 

 centre of the body, and represents the original rudiments 

 of the spine or vertebral column. This is the notochord, 

 the chorda dorsalis, or chorda vertebralis, which is also called 

 the vertebral chord or spinal axis, or, briefly, the chorda. 

 This firm, but flexible and elastic chord, consists of a cartila- 

 ginous mass of cells, and forms the central inner axis of the 

 skeleton or main support of the body ; it occurs exclusively 

 in Vertebrates, and is entirely wanting in all other animals. 

 As the first rudiment of the spine, it possesses the same sig- 

 nificance in all Vertebrates, from the Amphioxus to Man 

 But in the Amphioxus alone the notochord is retained, 

 throughout life, in its simplest form. In Man and all the 

 other higher Vertebrates, on the contrary, it is found in this 

 form only in the earliest embryonic stages, and afterwards 

 develops into the articulated vertebral column. 



The spinal axis, or notochord, is the fixed main axis of 

 the Vertebrate body, corresponding with the ideal axis of 

 length, and at the same time serving as a sure guide by 

 which we learn the true bearing of the typical relative posi- 

 tions of the most important organs of the Vertebrate body. 

 By means of it we can picture the body of the Vertebrate in 

 its original natural arrangement, in which the axis of length 

 lies horizontally ; the dorsal side lies above, and the ventral 

 side below (Fig. 52). If we make a vertical section through 

 the whole length of this axis, the whole body separates into 

 two similar and symmetrical parts, the right and left halves. 

 In both halves exactly the same organs originally lie, in tho 



