572 



STRUCTURE OF VERTEBRATES 



in what is functionally a new part of the body, the neck. The head 

 of tetrapods thus corresponds only partially to that of fishes, and 

 no formal morphological definition of the term ' head ' seems 

 possible which will satisfy the facts in all vertebrates. 



THE CHONDROCRANIUM 



In all vertebrate embryos a large part of the skull is preformed 

 in cartilage, and in the Chondrichthyes the cartilaginous state 

 persists ; although it is probable that the absence of bone is 

 secondary, the skull of the dogfish does represent fairly well 



Nasalcapsule. 

 Optic capsuLe. 

 Hypophyseal 

 fenestra. 



Basitrabecular. 



Nasal sac- 



Tradecula . 

 Optic cup. _ 



PoLar cartilage. 



HypophijSis. 



Parachordal, 

 /^uditorysac. 



deromtrt. 

 Notochord. 



process. 



Acrochordai. 



'i. 



Auditori 

 'capsuL 



Occipital 

 arch- 



Fig. 446. — Diagrams showing the development of the chondrocranium in gnatho- 

 stomes, dorsal view. Some cranial nerves are shown, on the right side only. — 

 Simplified from Goodrich. 



the chondrocranium, or cartilaginous skull, which may be pre- 

 sumed to have preceded in evolution the bony skull. A similar 

 structure persists in modern Amphibia, although here a certain 

 number of bones is added. Such a chondrocranium is seen to 

 have an anterior ethmoid region, to the front of which is attached 

 a pair of nasal capsules separated by a nasal septum ; an orbito- 

 temporal region more or less compressed to hold the eyes and 

 having many foramina for the passage of nerves ; an otic region, 

 to which are applied laterally the paired auditory capsules ; 

 and a posterior occipital region with a more or less constricted 

 exit, the foramen magnum, for the spinal cord. Together these 

 partly enclose the brain, but this is exposed at places (the 

 fontanelles) where the cartilage is incomplete (cf. Figs. 240 

 and 267). In the floor in the orbitotemporal region is the hypo- 



