xiii.] THE FROG. 169 



away from it in the Frog, and the like difference obtains 

 between all Vertebrata and all Arthropoda. 



Using the term skeleton, in its broadest sense, for the 

 framework which protects, supports and connects the various 

 parts of the organism, it consists in the Frog of four kinds of 

 tissue; the Horny, the Osseous, the Cartilaginous and the 

 Connective. Moreover, the hard parts are either developed 

 in the integument, constituting an exoskeleton, or they are 

 deeper seated and belong to the endoskeleton. 



Leaving aside a question that may arise as to the nature 

 of some of the cranial bones, the exoskeleton in the Frog is 

 almost absent, being represented only by the horny coating 

 of the calcar. 



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

 as in all the higher Vertebrata, 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 



