6 ANIMAL BIOLOGY. [Part I. 



to develop, that is, there is a change in relative dimensions of 

 parts. There is, for example, a well-marked hump in the back 

 of a frog. If now we take measurements from this point to the 

 tip of the snout and the end of the back in an old frog, we shall 

 find that the length of the body anterior to this point is not 

 quite twice the length behind it ; whereas in a minute frog, just 

 emerged from its tadpole state, the length in front is more than 

 three times as great as that behind. So that when metamor- 

 phosis is over there is still development. 



Let us now note some of the more important points in the 

 external characters of the fully-grown frog. 



The body is oval, without neck or tail. The head is large, 

 and within it can be felt a smooth bony mass, the skull. There 

 is a hump in the back, anterior to which the separate vertebra of 

 the back-bone can be felt. Behind this hump there is a smooth 

 rod of bone, the urostyle. The skull, vertebrae, and urostyle 

 constitute the axial skeleton. In the "region of the chest the 

 breast-bone (sternum) can be felt ; but there are no ribs. 



There are four legs, within which the bony supports may be 

 felt. The fore-legs are small, and are jointed to the shoulder- 

 girdle : the hind-legs large and powerful, and attached to the 

 hip-girdle. The bones of the limbs and the supporting arches or 

 girdles constitute the appendicular skeleton. In the fore-limb are 

 the following parts : brachium, or arm ; antibrachium, or fore- 

 arm; and manus, or hand. The corresponding parts of the 

 hind-limb are : femur, or thigh ; crus, or shank ; and pes, or foot. 

 These corresponding parts are said to be homologous. The 

 manus has four digits, and is not webbed. The pes has five long 

 digits, and is webbed. Thickened pads (callosities) are developed 

 beneath the joints of all the digits, there being an extra one 

 just at the base of the innermost digit of the pes, where there is 

 apparently the rudiment of a sixth toe. The part of the 

 manus answering to our wrist is short and small ; but the part 

 of the pes answering to our ankle is very much elongated so 

 much so as to give the appearance of an extra division of the 

 hind limb between the shank and the foot. 



The normal position of the limbs of the frog (Fig, 3, A) should 



