THE FORMS OF THE SKULL. 21 



becomes connected with Meckel's cartilage beliind, and with 

 the prefrontal cartilage in front. 



The maxillary process is, at first, separated by a notch 

 coiTCsponding with each nasal sac, from the boundary of 

 the antero-median part of the month, which is formed by 

 the ft-ee posterior edge of a fronto-nasal process (Fig. 4, F ; 

 Fig. 8, fc). This separates the nasal sacs, and contains the 

 cartilaginous, ethmovomerine, anterior termination of the 

 skull. The notch is eventually obliterated by the union of 

 the fronto-nasal and maxiUary processes, externally ; but it 

 may remain open internally, and then gives rise to the pos- 

 terior nasal aperture, by which the nasal cavity is placed in 

 communication with that of the mouth. 



The General Modifications of the Vertebrate Shull. — The 

 lowest vei-tebrated animal, Amphioxus, has no skuU. In 

 a great many fishes the development of the skuU 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 represented, on each side, by a cartilage answering to the 

 palato-pterygoid 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 Vertebrata, the cartilaginous cranium and 

 facial arches may persist to a greater or less extent; but 

 bones are added to them, which may be almost whoUy mem- 

 brane bones, as in the Sturgeon ; or may be the result of 

 the ossification of the cartilaginous cranium itself, from 

 definite centres, as well as of the development of superim- 

 posed membrane bones. 



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

 plete ossification, osseous matter is thrown down at not 

 fewer than three points in the middle of its cartilaginous 



