148 HISTORY OF THE HUMAN BODY 



(parasphenoid) , which forms almost the entire base of the 

 cranium in fishes and amphibians. Certain of these last named 

 do not develop, strictly speaking, in association with the cra- 

 nium, but are formed about certain elements of the visceral 

 skeleton, as will be explained below, but as these latter ele- 

 ments early lose their physiological independence and become 

 closely incorporated with the original chondrocranium, the 

 statement is in no way misleading. 



These dermal plates thus form an almost complete case 

 of bone, surrounding and protecting the internal cartilaginous 

 skull, and, by supplying the deficiencies of the latter, effect the 

 complete enclosure of the brain within the skeletal, parts. 

 There are thus formed two skulls, one within the other, and in 

 the ganoids, where the relation between the two is not as yet 

 a very intimate one, the outer or bony skull may be easily 

 removed from the other. 



The next step in advance is one shown also among the 

 ganoids, and consists of the strengthening of the chondrocra- 

 nium directly by the development of centers of ossification 

 within the cartilage itself (endochondral ossification) forming 

 definite osseous elements, called from their mode of origin 

 cartilage bones, in distinction from the other, the dermal 

 bones. Among these centers may be enumerated the exoc- 

 cipitals, the pro-otics, epiotics, and opisthotics, which together 

 form the petrosals, the ali-sphenoids, the orbito-sphenoids, and 

 the ethmoids, well-known elements in the skulls of higher 

 vertebrates, but here found at their inception, arising as iso- 

 lated areas of the chondrocranium and developing at the ex- 

 pense of the cartilage, clearly differing from the dermal bones 

 in origin. 



We thus find in the skull of the ganoids the elements of 

 the vertebrate skull almost at their beginning, and can trace 

 the origin of parts familiar to us as they appear in the spe- 

 cialized skulls of mammals, where, under the cloak of an 

 exactly similar external appearance, their diverse origin has 

 become lost. The ganoids seem thus a vital link in the story 

 of the skull, yet even had they become entirely extinct, as they 



