VERTEBRATE ANIMALS 



221 



PF 



which is a vessel joining the orbital to the anterior cardinal sinus. 

 A short distance along this is a foramen transmitting the ninth 

 cranial nerve, which comes from the floor of the capsule wherein it 

 runs, and at its posterior end another perforation just lateral of the 

 occipital condyles forms the exit for the vagus. 



The development of the cranium in the dogfish is interesting, 

 since it is similar in its early stages and main outlines to that of any 

 typical Craniate. The first skeletal structure in all these forms is, 

 as has already been pointed out, the notochord, which in Scyllium 

 runs under the brain as far forward 

 as the hinder end of the fore-brain, 

 i.e. up to the level of the pituitary 

 body. Two separate regions of de- 

 velopment of the cranium are con- 

 sequently to be noted, a posterior 

 chordal region, comprising the occipi- 

 tal and otic regions of the adult, and 

 an anterior pre-chordal portion later 

 forming the orbital and ethmoidal 

 regions. The first cartilages to appear 

 in the hinder region are two bars, the 

 par achor dais, one on each side of the 

 notochord, which subsequently fuse 

 around this to form the basal plate. 

 In its occipital region this shows 

 signs of segmentation, suggesting its 

 derivation from a modified anterior 

 part of the vertebral column,* but 

 there are no such indications in the 

 otic region. To the sides of this 

 latter lie the beginnings of the 

 membranous labyrinth, the auditory 

 vesicles, around which a cartilaginous 

 capsule is soon developed. In the 

 pre-chordal region also the first elements to appear are two carti- 

 laginous bars, the trabeculse cranii, and these two fuse to form a 

 trabecular plate. Their hinder ends, however, remain open for a 

 time, leaving the pituitary fontanelle, but later grow together below 

 the hypophysis cerebri, which thus comes to be lodged in a 

 depression in the cranial floor. At its front end the trabecular 

 plate is continued on to form the internasal septum. 



The side walls of the occipital region are formed by lateral 



* This suggestion is also supported by other evidence into which we cannot 

 enter here. 



FIG. 71. Development of cra- 

 nium in a Craniate, adapted 

 from Ziegler's model. 



A., auditory capsule ; N., notochord ; 

 O., occipital region : P., parachordal ; 

 PF., future position of pituitary fon- 

 tanelle ; T., trabecula cranii. 



