THE SKULL OF VERTEBRATES. 373 



which replace them, contribute directly to the building up of 

 the skull. 



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 

 displaced and modifi'ed 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. 



(d) When a bone develops in direct relation to a pre- 

 existent cartilage which it replaces, it is often called a 

 primary or " cartilage " bone. Thus many regions of the 

 brain-box which are cartilaginous in the embryo are replaced 

 by bones in the adult. 



But there are other bones which develop independently 

 of pre-existent cartilage. They invest the cartilaginous brain 

 box on the roof, on the floor {i.e., in the mouth cavity), 

 and on the sides. These are often called secondary or 

 " membrane " bones, and they seem to correspond to 

 dermal ossifications such as the bony parts of the skin-teeth 

 of Elasmobranchs, or to the bony plates of Ganoids. It is 

 likely that the roofing bones of the skull originated from plates 

 like those which form the dermal armour of sturgeons. In 

 structure, be it understood, a cartilage bone is not distin- 

 guishable from a membrane bone ; the distinction refers to 

 the development. 



To sum up, the skull is derived (a) from the parachordals 

 and the trabeculae at the end of the notochord, {b) from the 

 adjacent sense-capsules of nose and ear, {c) from the more 



