566 



THE PARACHORDALS AND NOTOCHORD. 



as to enclose a space the pituitary space. In front of this 

 space they remain in contact and generally unite. They extend 

 forwards into the nasal region (pn}. 



(3) The cartilaginous capsules of the sense organs. Of these 

 the auditory (ait) and olfactory capsules (ol} unite more or less 

 intimately with the cranial walls ; while the optic capsules, 

 forming the usually cartilaginous sclerotics, remain distinct. 



The parachordals and notochord. The first of these sets 

 of elements, viz. the parachordals and notochord, forming 

 together the basilar plate, is always an unsegmented continu- 

 ation of the axial tissue of the vertebral column. It forms the 

 floor for that section of the brain which belongs to the primitive 

 postoral part of the head (vide p. 314), and its extension is 

 roughly that of the basioccipital of the adult skull. Its mode of 

 development is almost identical with that of the vertebral 

 column, except that the notochord, even in many forms where it 

 persists in the vertebral column, disappears in the basilar plate ; 

 though in a certain number of cases remnants of it are found in 

 the adult state. 



It will be convenient to say a few words 

 notochord in the head. It always extends 

 along the floor of the mid- and hind-brains, 

 but ends immediately behind the infundi- 

 bulum. The limits of its anterior extension 

 are clearly shewn in fig. 43. The front end 

 of the notochord often becomes more or 

 less ventrally flexed in correspondence with 

 the cranial flexure ; its anterior end being 

 in some instances (Elasmobranchii) almost 

 bent backwards (fig. 324). 



Kolliker has shewn that in the Rabbit 1 , 

 and I believe that a more or less similar 

 phenomenon may also be observed in Birds, 

 the anterior end of the notochord is united 

 to the hypoblast of the throat 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. 



here with reference to the 

 nib 



FIG. 324. LONGITUDINAL 

 SECTION THROUGH THE BRAIN OF 

 A YOUNG PRISTIURUS EMBRYO. 



cer. commencement of the cere- 

 bral hemisphere; pn. pineal gland ; 

 ///.infundibulum ; //.ingrowth from 

 mouth to form the pituitary body ; 

 nib. mid-brain ; cb. cerebellum ; ch, 

 notochord; al. alimentary tract; 

 laa. artery of mandibular arch. 



" Embryologische Mittheilungen." Festschrift d. Nattirfor. G^//., Halle, 1879. 



