THEORY OF THE SKULL. 429 



ficial bony plates it is supposed that the " membrane " 

 bones or ossifications in membrane, which form so im- 

 portant 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 

 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 we 

 have 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 individual develop- 

 ment there is a parallel progress. 



Although one is safe in saying that skeletal structures in 

 Vertebrates are mostly mesodermic in origin, it should be 

 noted (i) that the notochord is endodermic, and (2) that in 

 the head certain ectodermic proliferations may give rise to 

 skeletal rudiments of a connective tissue nature which sub- 

 sequently become differentiated into cartilage (Goronowitsch, 

 Platt). But there is still doubt as to this last point. 



Theory of the Skull. 



Near the beginning of this century, Oken and Goethe independently 

 propounded what is known as the vertebral theory of the skull. Regard- 

 ing the skull as an anterior portion of the vertebral column, composed 

 of three or four vertebrae, they compared the bones of the different 

 regions to the parts of a vertebra. Thus in the hindmost region of the 

 skull, the basi-occipital, the two ex-occipitals, and the supra-occipital 



