24 THE ANATOMY OF YERTEBRATED ANIMALS. 



The General Modifications of the Vertebrate Skull. — The 

 lowest vertebrated animal, Amphioxus^ has no skull. In a 

 great many fishes, the development of the skull carries it no 

 further than to a condition which is substantially similar to 

 one of the embryonic stages now described ; that is to say, 

 there is a cartilaginous primordial cranium^ with or without 

 superficial granular ossifications, but devoid of any proper 

 cranial bones. The facial apparatus is either incompletely 

 developed, as in the Lamprey ; or, the upper jaw is repre- 

 sented, on each side, by a cartilage answering to the palato- 

 pter^^goid and part of Meckel's cartilage, while the larger, 

 distal portion of that cartilage becomes articulated with the 

 rest, and forms the lower jaw. This condition is observable 

 in the Sharks and Rays. In other fishes, and in all the higher 

 Vertehrata, the cartilaginous cranium and facial arches may 

 persist to a greater or less extent ; but bones are added to 

 them, which may be almost wholly membrane bones, as in the 

 Sturgeon ; or may be the result of the ossification of the car- 

 tilaginous cranium itself, from definite centres, as well as of 

 the development of superimposed membrane bones. 



The Osseous Brain-case. — When the skull undergoes com- 

 plete ossification, osseous matter is thrown down at not fewer 

 than three points in the middle of its cartilaginous floor. The 

 ossific deposit, nearest the occipital foramen, becomes the hasi- 

 occipital bone ; that which takes place in the floor of the pitu- 

 itary fossa becomes the hasisphenoid ; that which appears in 

 the reunited trabeculee, in front of the fossa, gives rise to the 

 presphenoid. Again, in front of, and outside, the cranial cav- 

 ity, the ethmoid may be represented by one or more distinct 

 ossifications. 



An ossific centre may appear in the cartilage on each side 

 of the occipital foramen, and give rise to the ex-occipital ^ and 

 above it, to form the supra-occipital. The four occipital ele- 

 ments, uniting together more or less closely, compose the oc- 

 cipital segment of the skull. 



In front of the auditory capsules and of the exit of the 

 third division of the fifth nerve, a centre of ossification maj 

 appear on each side and give rise to the alisphenoid ; which, 

 normally, becomes united below with the basisphenoid. 



In front of, or above, the exits of the optic nerves, the 

 itrhitosphenoidal ossifications may appear and unite below 

 with the presphenoid. 



In front of the occipital segment, the roof of the sloill is 

 formed by membrane ; and the bones which complete the two 



