THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 103 



c) Membrane bones of the gar pike: (Fig. 34!)) The head is prolonged into 

 a long snout having the nasal openings at its extremity. The small nasal bones 

 surround the nasal openings. Posterior to these occupying the center of the 

 dorsal surface of the snout are the elongated ethmonasal bones. Posterior to 

 them are the frontals, whose anterior ends inclose the posterior ends of the 

 ethmonasals. Posterior to the frontals are the large parietals and behind them 

 a number of temporals. The edges of the upper jaw are composed of the maxillae, 

 bearing teeth and each consisting of a series of squarish bones. The lower jaws 

 consist chiefly of the long dentary bones, bearing teeth. The sides of the head 

 behind the orbit are covered by a large number of small cheek plates and, posterior 

 to them, by the larger operculars. 



d) Membrane bones of the bowfin: The skull of Amia is "perhaps less special- 

 ized than in any other living teleostome, " according to L (Fig. 34^!). Identify 

 on the skull anterior stalked and posterior nostrils and the orbits. Between the 

 anterior nostrils projects the small triangular dermal mesethmoid, a membrane 

 bone. Anterior to this are the premaxillae, bearing teeth. Covering the space 

 between the four nostrils are the nasals, posterior to them in the median line the 

 large frontals, posterior to them the parietals, and at the posterior end of the 

 skull the four temporals. Below and anterior to the orbit is the large lacrimal, 

 below the orbits the two suborbitals, and posterior to the orbit two large post- 

 orbitals. Forming the sides of the upper jaw are the tooth-bearing maxillae. 

 Between the dorsal, postorbital, and the parietal is the squamosal. The 

 operculum is supported by several opercnlar bones. 



e) Demonstration of the separate origin of chondrocranium and membrane 

 bones: Examine the demonstration specimen of the head of a ganoid fish in which 

 the incasing membrane bones have been loosened from the underlying chondro- 

 cranium. Remove the sheath of membrane bones, noting that they are situated 

 in the skin, and note the chondrocranium, similar to that of the dogfish, lying 

 within the sheath. Such a specimen shows clearly the origin of the skull from 

 two separate sources, the cartilaginous chondrocranium and the dermal scales. 



D. THE FORMATION OF THE CARTILAGE BONES OF THE SKULL AND THE 

 COMPOSITION OF THE COMPLETE SKULL 



The next and last step in the formation of the skull is the production of cartilage bones 

 in the chondrocranium, including the sense capsules, and in the mandibular and hyoid arches. 

 In definite regions of these structures centers of ossification that is, centers of bone forma- 

 tion arise. Each of these centers transforms a certain area of cartilage into bone, and 

 each such bone is primitively a cartilage bone of the skull. It has already been stated that 

 in the majority of vertebrates the chondrocranium, present in the embryonic stage only, 

 consists of a ventrally situated plate with neither lateral nor dorsal walls, except at the extreme 

 posterior end (Fig. 32). Consequently, the cartilage bones of the skull, formed in the chondro- 

 cranium, are likewise limited to the ventral and extreme posterior parts of the skull, where 

 tbev form a floor for the brain. The roof and sides of the skull are completed by membrane 



