428 STRUCTURE OF VERTEBRATA. 



branous cranium, the arches also become chondrified, and 

 at the same time divided into successive segments. 



Of these arches, there are never more than eight. The 

 most anterior is the mandibular arch which bounds the 

 mouth, the second the hyoid these two are of great 

 importance in the development of the skull. The others, 

 in Fishes and at least young Amphibians, bound open gill 

 slits and support the pharynx ; above Amphibians, they 

 are less completely developed. 



In the Elasmobranch fishes, the mandibular and hyoid arches do not 

 form any direct part of the cartilaginous brain case, but in the Teleo- 

 steans and thence onwards, the cartilages, or bones, arising in connection 

 with the mandibular and upper part of the hyoid arches, contribute 

 directly to the formation of the skull. The hyoid proper, or lower 

 part of the hyoid arch, forms the skeleton supporting the tongue. 

 Cartilages arising in the lower part of the third visceral arch assist in 

 the formation of the hyoid bones of the higher Vertebrates, and parts of 

 two other arches appear to help in forming the laryngeal skeleton of 

 Mammals. 



The mandibular arch in Elasmobranchs and frogs divides into a lower 

 portion Meckel's cartilage which forms the lower jaw or its basis, 

 while from the upper portion a bud grows forward the palato-pterygo- 

 quadrate cartilage, which forms the upper jaw in shark and skate, and 

 has a closer union with the skull in the frog. In higher Vertebrates, the 

 lower portion of the mandibular always forms the basis of the lower 

 jaw, a quadrate element is segmented off from the upper part, 

 but the palato-pterygoid part seems to arise more independently. The 

 hyoid arch also divides into a lower portion, the hyoid proper, and an 

 upper portion, the hyo-mandibular, which may connect the jaws with 

 the skull, or from Amphibians onwards may be more remarkably dis- 

 placed and modified as a columella or stapes connected with the ear. 

 We adhere to the old interpretation, according to which the mandibular 

 and hyoid form two arches ; even if Dohrn's theory that they are 

 equivalent to four be accepted, the general fact remains that certain 

 arches aid in the development of the skull. 



Returning now to the brain box itself, we must notice 

 another complication, the development of " membrane " 

 bones. If we examine the skull of the skate, we find that 

 the brain lies within a cartilaginous capsule, but this is not 

 entirely closed, spaces (the fontanelles) being left in the 

 roof, which during life are covered only by the tough skin 

 with its numerous " dermal denticles." In the sturgeon, 

 again, the small skin teeth are replaced by stout bony plates 

 covering over the cartilaginous capsule. From such super- 



