THE SKELETON 433 



(c) in the orbital region: the basisphenoid, the orbitosphenoids, 

 the ahsphenoids and ossifications of the interorbital septum; (d) in 

 the ethmoidal region the bony ethmoidal skeleton; (e) the palato- 

 quadrate cartilage furnishes the quadrate bone; (/) a proximal 

 ossification, the articulare, arises in Meckel's cartilage and fuses 

 later with membrane bones; (g) the upper part of the hyoid arch 

 furnishes the columella, and the ceratohyals the os entoglossum; 

 (h) the cerato- and epibranchials ossify independently, as also 

 do the two copulse. (See Figs. 243, 244 and 245.) 



The membrane bones of the skull are: (a) in the region of the 

 cranium proper: parietals, frontals, squamosals; (6) in the facial 

 region: lachrymals, nasals, premaxillae, maxillae, jugals, quad- 

 rato-jugals, pterygoids, palatines, parasphenoid, and vomer; (c) 

 surrounding Meckel's' cartilage and forming the lower jaw: angu- 

 lare, supra-angulare, operculare, and dentale. (See Figs. 243, 244 

 and 245.) 



The embryonic bird's skull is characterized by a wealth of 

 distinct bones that is absolutely reptilian; but in the course of 

 development these fuse together so completely that it is only in 

 the facial and visceral regions that the sutures can be distinguished 

 readily. 



In order of development the membrane bones precede the 

 cartilage bones, though the latter are phylogenetically the older. 

 Thus, about the end of the ninth day, the following bones are 

 present in the form of delicate reticulated bars and plates: all 

 four bones of the mandible, the faint outline of the premaxillae, 

 the central part of the maxillae, the jugal and quadratojugal, the 

 nasals, the palatines and pterygoids. The base of the squamosal 

 is also indicated by a small triangular plate ending superiorly in 

 branching trabeculae, delicate as frost-work. A faint band of 

 perichondral bone is beginning to appear around the otic process 

 of the quadrate, the first of the cartilage bones to show any 

 trace of ossification. These ossifications appear practically 

 simultaneously as shown by the examination of the earlier stages. 



On the twelfth day these areas have expanded considerably, 

 and the frontals and prefrontals (lachrymals) are formed; the 

 rostrum of the parasphenoid is also laid down, and the exoccipi- 

 tals appear in the cartilage at the sides of the foramen magnum. 

 The parietals appear behind the squamosal (Fig. 242) about the 

 thirteenth day; the basioccipitals soon after. The supraoc- 



