144 



ON THE STRUCTURE OF THE SKULL. 



from two others which appear, not in cartilage, but in the mem- 

 branous roof of the skull above the limits of the cartilage, and 

 so give rise to that part of the squama occipitis marked SO'. 



The basi-sphenoid is developed from two centres of ossifica- 

 tion which appear in the floor of the sella turcica, but speedily 

 coalesce into one. Two separate centres of ossification appear 

 in the cartilage between these and the alisphenoids, and form 

 the lingulm sphenoidales. Each alisphenoid is developed from a 



Fie. 59. 





Fig. 59. — A, upper, and B, under view or" the basis cranii and periotic cartilage of a 

 human fcetus eight inches long. The alisphenoidal and immediately adjacent parts of 

 the basi-sphenoid are omitted. The cartilage is darkly shaded, while the osseous de- 

 posits are left white, or but lightly shaded. The greater part of the supra-occipital 

 is cut away. The clear spaces close to the dotted lines leading from t.tj are apertures 

 in the cartilage. The epiotic classification has not yet appeared, and the pro-otic and 

 opisthotic ossifications are quite distinct on the right side. 



