594 MEMBRANE BONES. 



one row lying at the edge of the mouth, on the outer side of the 

 pterygo-palatine process, and the other set on the roof of the 

 mouth superficial to the pterygo-palatine process. 



The outer row is formed of the prcemaxilla, maxilla, jugal, 

 and very often quadrato-jugal. Of these bones the maxilla 

 and praemaxilla, as is more especially demonstrated by their 

 ontogeny in the Urodela, are partly derived from dentigerous 

 plates and partly from membrane plates outside the mouth ; 

 while the jugal, and quadrato-jugal when present, are entirely 

 extra-oral. In the Amphibia and Amniota the praemaxillae and 

 maxillae are the most important bones in the facial region, and 

 are quite independent of any cartilaginous substratum. 



The second row of bones is clearly constituted in the Dipnoi 

 and Amphibia by the vomer in front, then the palatine, and 

 finally the pterygoid behind. Of these bones the vomer is 

 never related to a cartilaginous tract below, while the palatines 

 and pterygoids usually are so. The position and growth of the 

 three bones in many Urodela (Axolotl) are especially striking 

 (Hertwig. No. 442). In the Axolotl they form a continuous 

 series, the vomer and palatine being covered by teeth, but the 

 pterygoid being without teeth. The vomer and palatine origi- 

 nate from the united osseous plates of the bases of the teeth, 

 while the pterygoid is in the first instance continuous with the 

 palatine. 



In Teleostei, Amia, etc., there are dentigerous plates forming 

 a palatine and pterygoid, which in position, at any rate, closely 

 correspond with the similarly named bones in Amphibia; and 

 there is also a dentigerous vomer which may fairly be considered 

 as equivalent to that in Amphibia. 



In the Amniota the three bones found in Amphibia are always 

 present, but with a few exceptions amongst the Lacertilia and 

 Ophidia, are no longer dentigerous. The cartilaginous bars, 

 which in the lower types are placed below the palatine and 

 pterygoid membrane bones, are usually imperfectly or not at all 

 developed. 



On Meckel's cartilage important membrane bones are almost 

 always grafted. On the outside and distal part of the cartilage a 

 dentary is usually developed, which may envelope and replace 

 the cartilage to a larger or smaller extent. Its oral edge 



