THEORY OF THE SKULL. 433 



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 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 — (1) 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, 

 Piatt). 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. Regarding 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 were held to correspond to the centrum, the 

 neural arches, and the neural spine of a vertebral body. 



This undoubtedly suggestive theory, modified in various details, per- 

 sisted for a long period, but ultimately gave way before the advances in 

 comparative anatomy and embryology. Huxley gave it its deathblow, 

 and Gegenbaur replaced it by what may be called the segmental theory 

 of the skull. 



To realise this theory we must go back in development to the period 

 before the mesoblasl has ensheathed the notochord. At this time the 

 segmentation of the body is expressed, not in the skeleton (notochord), 

 but in the primitive segments. The segments, though less obvious 

 than in the trunk, are represented in the head region. Formerly nine 

 were enumerated, but it appears that in Elasmobranchs they are 

 more numerous. Subsequently brain and spinal cord become alike 

 enveloped in the mesoblastic sheath, which gives rise to the skeleton of 

 both head and trunk. 

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