I04 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



by resorption of the cartilage. The trabecute are at first distinct from each 

 other as well as from the perichordal part; later they fuse in front of the hypo- 

 physis to give rise to the base of the interorbital septum. In Tinnunculus the 

 ethmoid plate arises early as an intertrabecular mass, from which, later, the 

 dorsal part of the interorbital septum arises as a backward growth of cartilage. 



Fig. io8. — Earlier and later stages of skull of bird {Tinnunculus) after Suschkin. 

 ol, alisphenoid cartilage; ai, foramen for internal ophthalmic artery; b, basal plate; 

 bf>t, basipterygoid; ec, external semicircular canal; hm, 'hyomandibular; iorb, interor- 

 bital plate; itr, intertrabecula; mc, middle concha of nose; ot, octi capsule; ov, occipital 

 vertebrae; pc, posterior semicircular canal; sorb, supraorbital; sir, supratrabecula ; 

 Ir, trabecula. 



Large alisphenoid cartilages are connected with the otic capsules. The nasal 

 capsules are complicated and later give rise to several centres of ossification. 

 The quadrate is free from the rest of the cranium (streptostylic) and its ptery- 

 goid process, the homologue of the pterygoid cartilage in other groups, is greatly 

 reduced. The other visceral arches are much as in the adult (infra). 



Fig. 



109. — Diagram (after Boas) of the movement of the upper jaw of birds, 

 tine; /, pterygoid; z, zygomatic arcade. 



/, pala- 



The bones are lighter than those of reptiles and are often pneumatic, that is, 

 are penetrated with canals connected with the respiratory system. The brain 

 cavity is larger than in reptiles; sutures between the bones largely disappear in 

 the adult, and the single occipital condyle (mostly basioccipital) is on the floor of 



