210 



THE FROG. 



cartilaginous skeleton ; and (ii) membrane bones, which arise 

 independently of the cartilaginous skeleton, although they may 

 in their later stages become firmly grafted on to this. 



The membrane bones of the skull appear in a connective- 

 tissue layer, very rich in cells, which is found immediately 

 outside the cartilaginous cranium. In this connective tissue, 

 trabeculas and spicules of calcified substance appear, and soon 

 interlace to form a network ; surrounding the spicules are osteo- 

 blasts or bone cells, by which the further growth of the framework 



is effected, a consistent bony 

 lamella with imbedded bone cells 

 being ultimately formed. 



The first bone to be developed 

 is the parasphenoid, which ap- 

 pears, in tadpoles of about 

 20 mm. length, in the connective 

 tissue underlying the floor of the 

 skull. The exoccipitals, and the 

 frontals and parietals, which are 

 at first separate from one another, 

 soon follow. The premaxilla, 

 maxilla, dentary, and angulare 

 are formed at the commencement 

 of the metamorphosis; and 

 towards its close the vomer, 

 palatine, pterygoid, and other 

 bones appear. 



By the time the metamorphosis is completed, and the tail 

 absorbed, all the bones of the skull are present except the 

 sphen-ethmoid, which appears rather late, in the course of the 

 first summer, as a narrow transverse splinter of bone, crossing 

 the roof of the skull near its anterior end. 



3. The Teeth. 



In the frog the teeth are confined to the preniaxilla?, maxillae, 

 and vomers, the lower jaw being edentulous. 



The upper jaw bears a single row of small conical teeth 

 arranged along its inner border, each tooth being attached to 

 the bone at its base and along its outer surface, and only a very 

 small part of the tooth projecting freely. 



FIG. 95. The skull of an adult 

 Frog, from the ventral surface. 



x 11 



x i 2 . 



a, parasphenoid. c, columella. 

 e, exoccipital. f.p, fronto-parietal. 

 m, maxilla. , vomer. o, pro-otic. 

 p, pterygoid. pa, palatine, pm, pre- 

 maxilla. (/, quadrato-jugal. #?, 

 csphen-ethmoid. 



