124 HISTORY OF THE HUMAN BODY 



as high as ganoids, it retains much of its original appearance 

 and function. Both in Amphioxus and in these forms, as well 

 as in all embryos, it is formed of a semi-gelatinous tissue, 

 often called pre-cartilage , and is surrounded by a firm sheath 

 of connective tissue, which, in those adult forms with a per- 

 sistent notochord, supplies the necessary firmness and rigidity 

 in which particular the notochord alone would be inadequate. 

 This sheath becomes, in fact, of far greater importance than 

 the notochord, and the cartilaginous and osseous tissues 

 formed from -it come to encroach more and more upon the 

 yielding tissue within, and eventually supply the main elements 

 used in the construction of the vertebrae. 



The first stage in this advance is seen in the lamprey and 

 other cyclostomes, where there appear pairs of little cartilages, 

 lying upon the side of the notochord sheath and projecting 

 upwards to protect the nerve cord, which lies along the dorsal 

 side of the notochord. These little cartilages are of two kinds. 

 The primary ones develop from the edges of the intermuscular 

 septa, and are hence intersegmental, a point which is important 

 to remember in connection with the relative position of the 

 vertebrae in higher forms. A secondary set alternate with 

 these, and form intercalary pieces, protecting the intervals 

 between the first set. 



A second advance over the condition found in Amphioxus 

 is seen in the formation of a head, which, since here the noto- 

 chord no longer extends to the tip of the snout but ends a 

 little behind the plane of the eyes, has been supposed to be in 

 part a transformation of the anterior end, and in part a new 

 formation added anterior to this. Whether this may be safely 

 assumed or not; the anterior termination of the notochord 

 forms in the embryo an important topographical point, the 

 portion of the head along the sides of the notochord being 

 referred to as parachordal, and that anterior to it as pra- 

 chordal. The hypophysis, an organ lying in the median line 

 and depending from the lower surface of the brain, lies at the 

 anterior point of the notochord, and will thus serve to mark 

 the boundary between these two portions of the head. 



