12 ELEMENTARY BIOLOGY. [CHAP. 



The endoskeleton, on the contrary, is well developed and, 

 as in all the higher Vertebrata, may be resolved into an 

 axial and an appendicular portion. 



The adult axial endoskeleton is represented by the spinal 

 column and the skull. 



The appendicular endoskeleton consists of the limb-bones 

 and the pectoral and pelvic arches to which they are at- 

 tached. 



In the order of development, the endoskeleton is at first 

 represented by a membranous rod or notochord alone; 

 secondly, nascent connective tissue and cartilage are super- 

 added to the notochord ; thirdly, these acquire their special 

 characters ; fourthly, they become replaced by bone, wholly 

 or in part. 



The processes of conversion and replacement indicated 

 under the last head are very incomplete, even in the adult 

 Frog, for remains of the notochord are to be found in the 

 centres of the vertebrae (intra-vertebral bodies) ; while the 

 cartilage, of which the greater part of the skeleton was at 

 one period of larval existence composed, to a great extent 

 persists. 



Such cartilage is found forming the free surfaces of the 

 bodies of the vertebrae, the extremities of the caudal style 

 (urostyle) and the ends of the transverse processes ; and it 

 enters largely into the girdles. In the skull, nearly all the 

 bony elements may be removed, leaving behind the primi- 

 tive cartilaginous skull, or Chondro-cranium, altered only so 

 far as parts of it have been replaced by bone. 



It furnishes a floor, side walls and roof to the brain-case, 

 interrupted only by three spaces (fontanelles) in its roof 

 covered in by membrane, and by the foramina for exit 

 of the cranial nerves. In front it is continued forward 



