THE DEVELOPMENT OF THE HUMAN SKULL. I 17 



cranial arches, arc primarily developed in cartilage, only one of 

 the superior elements of these arches — the supra-occipital (SO) 

 — is so ; while the upper or " interparietal ' portion of the 

 squama ocevpitis (SO 1 ) and the two other pairs of superior 

 elements of the arches are developed altogether from mem- 

 brane. 



Fig. 60. 



Aa.jjt^' '. 



S.O* 



M a MO. 



KiiT. 60. — Longitudinal and vertical section of the basis cranii of a foetus somewhat older 

 than the foregoing (Fig. 59). The basi-sphenoidal and presphenoidal centres have 

 coalesced ; but they and the basi -occipital are severally separated by wide interstices 

 of cartilage, of which the whole ethmoidal region is still constituted. 



The ethmoid is developed from a single centre, arising in 

 the internasal cartilage. Its so-called lateral masses, with the 

 two upper spongy bones, are likewise developed each from a 

 single centre within the superior part of the inflected lateral 

 cartilages which wall in the olfactory sacs. The inferior tur- 

 binals are ossifications of the lower parts of these cartilages. 

 But the nasal bones are developed within the perichondrium, 

 which is continuous with the membrane in which the frontal 

 bones are developed, and the vomer is produced within the 

 perichondrium on the under-surface of the internasal septum. 

 The bones of Berlin are also said to be developed from mem- 

 brane — the perichondrium of the presphenoidal cartilage, or the 

 walls of the olfactory sacs. 



The development of the temporal bone is particularly worthy 

 of attentive consideration. The squamosal and the tympanic 

 elements are developed in membrane, and, at first, lie perfectly 

 loose in this membrane, upon the outer side of the periotic 

 cartilage. The tympanic is a delicate ring, open above; the 

 squamosal is a mere rod, the zygoma, with an expanded posterior 



L 2 



