VIII. 



THE ENDOSKELETON: THE COMPARATIVE ANATOMY OF 

 THE SKULL AND THE VISCERAL SKELETON 



The skull or cranium is that part of the endoskeleton which is found within the head 

 where it covers and protects the brain. The skull is the most complex of all the parts of the 

 endoskeleton, owing in part to the fact that it is derived from several different sources. To 

 determine the homology of the bones of the skull of different vertebrates is a tremendous 

 task which is not yet completed. The earlier investigators named bones in different skulls 

 by the same names without any adequate evidence that they really were homologous, with 



olfactory sac 

 nasal capsule 



prechordal 



olfactory sac 



parachordal 



vertebral 

 sclerotome 



vertebral 

 sclerotome 



A B 



Fig. 31. — Diagrams to illustrate the development of the chondrocranium from the prechordals, 

 parachordals, sense capsules, and sclerotomes of the vertebrae of the occipital region. A, early stage 

 with the various cartilages separate. B, later stage, showing union of the prechordals with each other 

 and with the olfactory capsules to form the ethmoid plate, and union of the parachordals with each 

 other and with the otic capsules and occipital vertebrae to form the basal plate. (Combined from 

 figures by Goodrich and Wilder.) 



resulting confusion. In the following pages an attempt is made to facilitate the study of 

 the skull by analyzing the skull into its various components and tracing in the phylogenetic 

 scale the union of these components into the complete skull. 



A. THE CARTILAGE STAGE OF THE SKULL 



i. Origin of the chondrocranium. — The skull arises in the mesenchyme of the head. The 

 first step in the formation of the skull is the production of cartilage by,the mesenchyme. Five 

 pairs of cartilages appear. One pair forms along the sides of the anterior end of the notochord; 



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