DEVELOPMENT OF SKULL 



535 



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

 portion, the hyo-niandibular, 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 comphcation — 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 important an 

 element in the skull of the higherVertebrate, 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 with- 

 out injury 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, 

 fusing with the ossified remainder of the cartilaginous 

 cranium, form a complete bony capsule. In Cyclostomes 

 and Elasmobranchs the brain-box is wholly cartilaginous ; 

 above Elasmobranchs the cartilage is more or less 

 thoroughly replaced or covered by bones. In the in- 

 dividual development there is a parallel progress. 



The segmentation of the head, in contradistinction to the unseg- 

 mented skull, is expressed, although indistinctly, by the muscle seg- 

 ments and by the nerves supplying these, perhaps also by the lateral 

 sense organs, the ganglia, and the arches. 



