VIII. J THE PAUACHORDALS. 237 



behind where they embrace the front end of the noto- 

 chord, they diverge anteriorly for some little distance and 

 then bend in again in such a way as to enclose a space 

 — the pituitary space. In front of this space they again 

 unite and extend forwards into the nasal region. 



(3) The cartilaginous capsules of the sense organs. 

 Of these the auditory and olfactory capsules unite more 

 or less intimately with the cranial walls, while the optic 

 capsules, forming the sclerotics, remain distinct. 



The parachordals and notochord. The first of 

 these sets of elements, viz. the parachordals and noto- 

 chord, forming together the basilar plate, is an unseg- 

 mented continuation 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, and its extension is roughly that of the basiocci- 

 pital of the adult skull. 



Laterally it encloses the auditory sacs (Fig. 76), the 

 tissue surrounding these (forming the so-called ' periotic 

 capsules ') is in the chick never separate from the basi- 

 lar plate. In front it becomes narrowed, and at the 

 same time excavated so as to form a notch on each side 

 (Fig. 76, 5) through which the fifth nerve passes ; and 

 in front of this it again becomes expanded. 



In order to render our subsequent account more 

 intelligible, we may briefly anticipate the fate of the 

 basilar plate. Behind it grows upwards on both sides, and 

 the two outgrowths meet above so as completely to enclose 

 the medulla oblongata, and to circumscribe a hole known 

 as the ' occipital foramen.' And it is at this point only 

 that the roof of the skull is at any period formed of 

 cartilage. 



