466 



CONSTITUENTS OF THE CRANIUM. 



pn 



It will be convenient to describe, in the first instance, the develop- 

 ment of the cartilaginous elements of the skull. 



The Cranium. The brain is at first enveloped in a continuous 

 layer of mesoblast known as the membranous cranium, into the base 

 of which the anterior part of the notochord is prolonged for some 

 distance. The primitive cartilaginous cranium is formed by a differ- 

 entiation within the membranous cranium, and is always composed 

 of the following parts (fig. 823) : 



(1) A pair of cartilaginous plates on eaoh side of the cephalic 



section of the notochord, known 

 as the parachordals w^a. ch). 

 These plates together with the 

 notochord (wc) enclosed between 

 them form a floor for the hind- 

 and mid-brain. The continuous 

 plate, formed by them and the 

 notochord, is known as the ba- 

 silar plate. 



(2) A pair of bars forming 

 the floor for the fore-brain, known 

 as the trabecule (tr). These 

 bars are continued forward from 

 the parachordals. They meet 

 behind and embrace the front 

 end of the notochord ; and after 

 separating for some distance bend 

 in again in such a way as to 

 enclose a space — the pituitary 

 space. In front of this space 

 they remain in contact and gene- 

 rally unite. They extend for- 

 wards into the nasal region 

 {pn). 



(3) The cartilaginous cap- 

 sules of the sense organs. Of 



Fig. 323. Head of embryo Dogfish, 



SECONJ) STAGE ; BASAL VIEW OF CKANIUM FR)M 

 ABOVE, THE CONTENTS HAVING BEEN BEHOVED. 



(From Parker.) 



ol. olfactory sacs ; au. auditory capsule ; 

 nc. notochord; py. pituitary body; jxi.ch. 

 parachordal cartilage; tr. trabecula; inf. 

 infundibulum ; C.tr. cornua trabecule ; pn. 

 prenasal element ; sp. spiracular cleft ; br. 

 external branchiffi ; CI. 2, 4. visceral clefts. 



these the auditory (au) and ol- 

 factory 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 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 

 (vide p. 261), 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 



