'Sm THE FIFTH UAY. [CliAP. 



It will be convenient to say a few words here with reference 

 to the notochord in the head. It always extends along the floor 

 of the mid- and hind-brains, but ends immediately behind the 

 iufuudibulum. The front end of the notochord becomes more or 

 less veutrally flexed in correspondence with the cranial flexure ; 

 its anterior end being in some animals (Elasmobranchii) almost 

 bent backwards (Fig. 77). 



Fig. 77. 



Longitudinal Section through the Head op a young 

 Pristiueus Embryo. 



cer. commencement of the cerebral hemisphere, pn. pineal gland. 

 In. infundibulum. pt. ingrowth from mouth to form the 

 pituitary body. mh. mid-brain, cb. cerebellum, ck. noto- 

 chord. al. alimentary tract. Jan. artery of mandibular 

 arch. 

 KoUiker has shewn that in the Eabbit, and a more or less 

 similar phenomenou may also be observed in Birds, the anterior 

 cud of the notochord is united to the hypoblast of the thioat in 

 immediate contiguity with the opening of the pituitary body ; 

 but it is not clear whether this is to be looked upon as the 

 remnant of a primitive attachment of the notochord to the hypo- 

 blast, or as a secondary attachment. 



Within the basilar plate the notochord often exhibits two or 

 more dilatations, which have been legarded by Parker and 

 KoUiker as indicative of a segmentation of this plate ; but they 

 hardly appear to be capable of this interpretation. 



