Skull of Frog. 



93 



These are relatively late appearances, and are greatly different 

 in number in the several types of the Vertebrata, whose skulls 

 we have to consider. 



Up to this point there is a great similarity in all skulls. 

 There is no wide divergence from this plan of structure and 

 development, though, of course, plenty of slight differences in 

 various details. The constitution of the adult skulls of frog, 



P.MX 



Fig. 39.— Skull of Frog. Dorsal aspect. (After W. K. Parker.) 

 The membrane bones are dotted ; the cartilage bones transversely shaded ; 

 the cartilage is left white. This applies also to Figs. 40 and 41. 

 B.0, basi-occipital region ; s.o, exoccipital bones ; peo.ot, pro-o ic ; 

 PTV, pterygoid; v, f, parieto frontal; na, nasal; p. MX, premaxiUa. 

 Jix, maxilla ; TEM, squamosal ; Qb, overlies position of quadrate 

 cartilage, and in front is quadrato-jugal (shaded). 



fowl, and rabbit differ principally in the modifications of the 

 visceral arches, in the number of the ossifications in the 

 primordial cartilage, and in the membrane bones surrounding 

 the skull. 



As the Frog's skull is on the lowest level, we will commence 

 with that. The absolute independence of the membrane bones 

 from the underlying, partly cartilaginous and partly ossified, 



