460 STRUCTURE OF VERTEBRATA. 



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 

 modified as a columella or stapes connected with the ear. 



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- 

 ficial bony plates it is supposed that the " membrane " 

 bones, or ossifications in membrane, which form so import- 

 ant an element in the skull of the higher Vertebrate, have 

 originated. 



In some bony fishes, notably the salmon, we find the brain enclosed 

 in a double capsule. Inside there is a cartilaginous brain-case in which 

 what are called centres of ossification have appeared, and upon this a 

 layer of membrane bones is placed, which can be readily removed without 

 iujury to the cartilage beneath. In general, however, we must recognise 

 that, with the appearance of membrane bones, two changes tend to 

 occur, first, the cartilaginous cranium tends to be reduced and to 

 exhibit considerable openings ; second, in the remaining cartilage 

 centres of ossification appear, and we thus have "cartilage" bones 

 formed. Further, in spite of the developmental differences, the mem- 

 brane and cartilage bones become closely united to one another, or 

 even fused, and there is thus formed "a firm, closed, bony receptacle 

 of mixed origin," as exemplified by the skull of any of the higher 

 Vertebrates. 



We may thus say that in the evolution of the skull there 

 is first a cartilaginous capsule, that this becomes invested 

 to a greater or less extent by dermal ossifications, and that 

 finally the dermal bones lose their superficial position, and, 



