330 THE CHICK. 



wards to form the side walls of the skull (Fig. 131) ; the ex- 

 occipital, ali-sphenoidal, and orbito-sphenoidal regions being 

 already established. 



In front of the pituitary body the ethmoidal plate (Fig. 131, 

 ET) has grown enormously ; it extends forwards to the tip of the 

 beak, and is fused in front with the cartilaginous capsules of the 

 olfactory organs, OK. From the dorsal surface of the ethmoidal 

 plate, along its whole length, a huge vertical crest, the interorbital 

 plate, has arisen, which supports the fore part of the brain (Fig. 

 11 G) along its upper edge, and is notched in front for the 

 passage of the olfactory nerves, I. 



(iii) The Visceral Skeleton. 



In the chick embryo, cartilaginous elements, corresponding 

 to the cartilaginous bars of the tadpole's skull, are developed 

 in the mandibular, hyoidean, and first branchial arches. 



The mandibular arch. In the mandibular arch two cartilages 

 appear, proximal and distal respectively, which are from the first 

 independent. The proximal, or dorsal, one (Fig. 131, Q) is the 

 quadrate cartilage, a stout tri-radiate cartilage of which the 

 longest arm, or otic process, is directed backwards, and articu- 

 lates with the auditory or periotic capsule ; while the ventral, 

 and stoutest limb furnishes the articular surface for the mandible. 

 The distal cartilage of the mandibular arch is a slender rod, 

 Meckel's cartilage, MC, which forms the basis of the lower jaw ; 

 its hinder end, AR, which articulates with the quadrate, is ex- 

 panded and thickened. 



The hyoid arch. The bar of cartilage belonging to the hyoid 

 arch is imperfect or absent along the greater part of its length, 

 its dorsal and ventral ends alone being present. The uppermost 

 or dorsal end is believed to be represented by the columella 

 (Fig. 131, CL), a slender rod of cartilage which very early fuses 

 with the stapes, a small plug of cartilage formed in the membrane 

 closing the fenestra ovalis. The ventral end of the hyoid bar 

 forms the cerato-hyal, or lesser cornu of the hyoid, HR ; and the 

 median element of the hyoid, or basihyal, appears also to belong 

 to this arch. 



The first branchial arch. In the ventral part of the first 

 branchial arch a slender cartilaginous bar, the cerate-branchial, or 

 greater cornu of the hyoid (Fig. 131, I?K), is formed ; and in the 



