XIII.] THE FROG. 165 



the bones and between them, in the glenoidal and acetabular 

 cavities. .' 



In like manner, the bones of the limbs consist originally 

 of merely cartilaginous models of the perfect bone ; but, as 

 development proceeds, the middle of the cartilaginous model 

 commonly becomes invested by a sheath of true bone, while 

 a calcareous deposit takes place in the cartilage close to its 

 growing extremities. As the bone grows, the superadded 

 sheath invades the middle of the cartilage and more or less 

 replaces it ; while the terminal portions of cartilage continue 

 to grow and enlarge and the calcareous deposit within them 

 increases, without however reaching their surfaces. Thus 

 one of the larger adult limb-bones, the femur, consists of 

 a median shaft of perfect bone, and of two terminal cones of 

 cartilage, containing calcified epiphyses, inclosed within and 

 more or less overlapping the hollow ends of the shaft. 



The general disposition of the parts which are seen in the 

 mouth has already been described. 



Teeth are found attached only to the premaxillse, maxilla3 

 and vomers. They are small, with recurved and pointed 

 crowns. New teeth are constantly being developed in the 

 gum to replace those which are worn out or broken away. 

 And as they attain their full size these teeth become anky- 

 losed to processes of the subjacent bone. 



The gullet passes without change of diameter into the 

 stomach, which lies on the left side of the abdominal cavity 

 and is nearly as long as it. The stomach narrows posteriorly 

 and the almost tubular pyloric portion bends round sharply 

 and passes into the duodenum. A slight constriction marks 

 the pylorus. The duodenum runs forwards parallel with the 

 stomach, so that with the latter it forms a sort of loop. At 

 its anterior end it passes into the rest of the small intestine 

 (ileum), which is coiled up into a sort of packet and lies 



