162 ELEMENTARY BIOLOGY. [XIII. 



almost absent, being represented only by the horny coating of 

 the calcan 



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

 as in all the higher Yertebrata, may be distinguished into an 

 axial and an appendicular portion. 



The axial endoskeleton consists of the notochord, the spinal 

 column and the skull. 



The appendicular endoskeleton occurs in the limbs and in 

 the pectoral and pelvic arches to which they are attached. 



In the order of development, the endoskeleton is at first 

 represented by the notochord alone ; secondly, nascent con- 

 nective tissue and cartilage are superadded to the notochord ; 

 thirdly, these acquire their special characters ; fourthly, they 

 become replaced by bone. 



The process of conversion or replacement indicated under 

 the last head is very incomplete, even in the adult Frog, 

 in which remains of the notochord are to be found in the 

 centres of the vertebrae; and the cartilage, of which the 

 greater part of the skeleton at .one period of larval ex- 

 istence was 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 sternum. In the skull, the para- 

 sphenoid 1 , vomers, parieto-frontals, nasals, premaxillse, max- 

 illae, jugals, squamosals, and the bony elements of the man- 

 dible may be removed by maceration, leaving behind the 

 primitive 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 so far as parts of it have been replaced by bone.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 a large space (called a fontanelle) covered 



1 See Laboratory work, D. C, for the structure of the bony skull (osteo- 

 cranium), which should be thoroughly understood before any attempt is made 

 to study the cartilaginous skull or chondro-cranium. 



