IX.] THE OSSIFICATIONS OF THE CRANIUM. 233 



but their anterior corners just project out in front of it 

 (Fig. 71, as). > 



The occipital condyle formed by the coalescence of the 

 two originally separate cartilaginous knobs, is seen just below 

 the notochord (Fig. 71, nc), and the supraoccipitals (Fig. 

 71, so) have coalesced above the occipital foramen. 



14. The ossifications of the cartilaginous skull, which 

 have commenced at this stage, are * : — 



(1) An ectosteal ossification around the notochord on 

 the inner surface of the skull immediately behind the 

 occipital condyle. This soon spreads to the opposite surface 

 and forms an ossifying centre for the basi occipital. 



(2) An ectosteal ossification (Fig. 71, eo) in each exoc- 

 cipital beginning immediately behind the vagus foramen 

 (Fig. 71, 8) ; this ossification commences on the exterior, but 

 soon spreads round to the inner surface of the cartilaginous 

 exoccipital. 



(3) The palatine (Fig. 71, pa) has become entirely 

 ossified by endostosis, the only instance of this process in the 

 ossification of the primordial skull of the bird. 



(4) An ectostosis of the pterygoid (Fig. 71, pg). 



(5) An ectosteal ossification of the quadrate (Fig. 71, q). 



(6) An ectosteal ossification of the cerato-branchials. 



15. At this stage the majority of the true membrane 

 bones also begin to be formed. 



The paired premaxillaries (Fig. 71, px) are formed in 

 the investment of the prenasal cartilage ; they are trian- 

 gular with the apex directed forwards, and soon begin to 

 develope their three normal processes : one above towards 

 the frontal (the nasal process), one along the margin of the 

 beak to join the maxillary (the marginal process), and one 

 below along the middle of the palate, to join the palatine 

 bones. The maxillaries (Fig. 71, mx) are develo]3ed on 

 each side outside the endoskeleton. Pointed at each end, 

 they are broader in the middle, sending a process inwards 



1 The term ectostosis or ectosteal is used when ossification sets in between the 

 perichondrium and the cartilage, endostosis when ossification takes place be- 

 tween the actual cells of the cartilage. These two processes not unfrequently 

 occur combined, when ossification by ectostosis on reaching the cartilage sets 

 up a true endosteal ossification. Parostosis is used for all ossifications which 

 take place in purely fibrous tracts. Parosteal products are often spoken of 

 as membrane or splint bones. 



