tilages appear ventral to the anterior ends of the parachor- 

 dals, and behind these the anlages of the palatoquadrate 

 and Meckel's cartilage. The jaws arise from a single mesen- 

 chymatous mass connected with the trabecula and the 

 anlage of the hyoid arch behind it. The attachment of the 

 anterior end of the palatoquadrate to the edge of the tra- 

 becula represents the basal process. With chondrification of 

 the several elements, there remains a mesenchymatous otic 

 process from the palatoquadrate up into the prespiracular 

 area. This process lies lateral to the head vein and the facial 

 nerve. 



The epihyal chondrifies from below upward; ventrally it 

 is in blastematic connection with the palatoquadrate — there 

 is no symplectic. A semiseparate pharyngohyal center 

 appears above the epihyal and fuses with it to form the 

 "hyomandibula." In rays this center fuses with the otic cap- 

 sule and forms the area of articulation of the epihyal; a large 

 part of the epihyal is formed from the fused bases of the car- 

 tilaginous gill rays. Centers for the cerato and basihyal are 

 also present. 



The parts of the auditory capsule are now joined laterally 

 and the capsule fused anteriorly with the underlying para- 

 chordal. The trabecula attaches to the polar cartilage, which 

 in turn soon joins the parachordal. The acrochordal forms 

 at the tip of the notochord and moves backward along the 

 notochord, in a relative sense, until it joins the parachordals. 

 The pila antotica joins the acrochordal laterally as well as 

 fusing with the parachordal behind it. The trigeminal, 

 abducens, and facial nerves emerge between this pillar and 

 the otic capsule. 



Between the otic capsules, the parachordals invade the 

 sheath of the notochord. The occipital arch has two or three 

 hypoglossal foramina penetrating it; there are separate 

 foramina for one or two dorsal roots of the more posterior 

 of these. One or two occipitospinal nerves pass out through 

 the metotic fissure with nerves IX and X. A chondrification 

 extends upward in the medial wall of the otic capsule, pos- 

 terior to the auditory nerves. The completed capsule has an 

 endolymphatic foramen, a canalicular foramen, and an 

 auditory nerve foramen in its medial wall. 



Anteriorly the trabeculae are joined by a plate from which 

 a rostral process projects. Laterally there are processes from 

 which separate orbitonasal laminae project dorsolaterally 

 and anteriorly. The orbital cartilage arches forward and 

 then down to attach to the trabecula by a preoptic pillar 

 passing medial to the orbitonasal division of the profundus 

 (Vi) and the superficial ophthalmic division of the facial 

 nerve. The orbitonasal cartilage lies lateral to the preoptic 

 pillar but soon fuses with it and the preorbital cartilage 

 which appears in the mesenchymatous ring margining the 

 orbit. Preorbital and postorbital centers appear in this ring 

 and later they are joined by a supraorbital cartilage; the 

 whole semicircle then fuses with the chondrocranium. The 

 line of union is marked by the foramina of the branches of 

 the superficial ophthalmic division of the facial nerve that 



serves the organs of the supraorbital sensory canal and the 

 skin. A posterior maxillary process extends back from the 

 ventral margin of the orbitonasal cartilage. 



The prootic foramen is formed by an orbitocapsular 

 commissure which is accompanied by a posterior extension 

 of the supraorbital-postorbital band. From the pila antotica 

 an extension divides the prootic foramen into upper and 

 lower openings. The lower closes and the upper is divided by 

 a commissure from the capsule into a trigemino-facio-ab- 

 ducens foramen and a posteroventral palito-hyomandibular 

 foramen. The abducens foramen later separates from the 

 others. The basal plate extends up between the hyomandib- 

 ular and palatine nerves. 



Anteriorly the rostral process, which represents the nasal 

 septum, expands along its dorsal margin into a trough-like 

 plate underlying a cavity, the precerebral fossa. This is 

 filled with a gelatinous connective tissue and separated from 

 the cranial cavity by a transverse membrane. The lateral 

 margins of the precerebral fossa connect with the orbitonasal 

 cartilage and with the sphenethmoid commissure of the 

 orbital cartilage. 



The orbit area is completed by closure of the large optic 

 fenestra. Several foramina are formed in this closure: optic, 

 oculomotor, efferent pseudobranchial, and pituitary vein. 

 An optic pedicel arises in the connective tissue attaching the 

 eye to the orbit wall. Ventrally the hypophyseal fenestra 

 closes from in front back, involving a plate between the 

 trabeculae. There is a scleral cartilage within the wall of 

 the eye. Posteriorly the parachordals are interconnected by 

 a thin sheet below the notochord. The notochordal sheath 

 itself is chondrified. 



The mesenchymatous lateral commissure gives rise to two 

 small prespiracular cartilages, a transitory procartilaginous 

 rod lateral to the palatine nerve, and a small extraotic car- 

 tilage which fuses to the capsule wall to form the canal for 

 the otic division of the facial nerve. 



The roof develops first from the synotic tectum, which 

 expands forward and posteriorly. Behind the endolymphatic 

 fossa a slim posterior tectum arises from the occipital arch, 

 which is now attached to the otic capsule above the metotic 

 foramen. Above the eyes, there are orbital, paraphyseal, 

 and epiphyseal tectal processes. These join and expand 

 until the roof is solid. 



The upper and lower lip cartilages are derived from the 

 blastema of the palatoquadrate; the posterior portion be- 

 comes the lower lip cartilage. The lower, small cartilage of 

 the upper lip arises from the mesenchymatic area of attach- 

 ment of the palatoquadrate to the trabecula. Posteriorly 

 there is a small blastema lateral to the quadrate area. A 

 mesenchymatous or chondrified intermandibular arises 

 between the Meckel's cartilages but is later lost. 



The shark lacks bone in its head skeleton, except for that 

 in the basal plates of the scales, but it has been suggested 

 that the mesenchymatous condensations giving rise to the 

 orbit margin and labial cartilages are indeed remnants or 



122 



THE HEAD SKELETON OF FISHES 



