2 THE FROG 



the mirrored image of the other. Notice the soft, moist skin and the 

 absence of hair, feathers, and scales. What is the relation of such a 

 skin to the habitat of the frog? Do you think the frog can live in 

 dry places? Observe the distribution of color on the skin. Are the 

 right and left sides colored alike? Are the dorsal and ventral sur- 

 faces colored alike? Do you know of any other animals that have 

 a comparable distribution of pigment? The body of the frog, like 

 that of many other animals, has three divisions: the head, the trunk, 

 and the appendages. Is the head of the frog sharply separated from 

 the trunk? For what is the long, flexible neck of birds and mammals 

 used? What structure has the elephant which compensates for its 

 short neck? Find the mouth, the anus, or posterior opening of the 

 digestive tract, the external nares, or nostrils, and the eyes. Compare 

 the eyelids with those of the human eye. Just posterior to the eye 

 is the tympanic membrane, or eardrum. Is the ear of the frog similar 

 to your own? The fore and hind limbs, or appendages, have the same 

 general structure; upper arm, forearm, wrist, and hand correspond to 

 thigh, shin, ankle, and foot. Consider how this similar fundamental 

 structure is modified for quite different purposes in the two pairs of 

 limbs. Compare with the limbs of a human being and other familiar 

 vertebrates. 



(c) Place the frog, dorsal surface up, in a dissecting pan, pinning 

 out the fore and hind limbs with their digits separated. Draw on a 

 scale of 1 as thus seen from the dorsal aspect, applying the principles 

 of scientific drawing explained on p. xxix. Show all the parts to which 

 your attention has been called in the foregoing section, if they appear 

 in this view. Finish the drawing in simple outlines, without shading, 

 and label carefully. 



B. The Body Wall and C(elom 

 Exercise 3.— The Body Wall. 



(a) Fasten the frog, ventral surface up, in a dissecting pan by 

 means of a pin thrust obliquely through each limb. Lift the skin 

 with the forceps and cut through it, but not through the underlying 

 muscles, with the scissors along the mid-ventral line from the pos- 

 terior end of the trunk to the tip of the jaw. The skin is separated 

 from the muscles except at certain places, and the spaces between 

 the skin and the muscles contain a colorless fluid called lymph. Cut 

 the skin outward, at right angles to the first cut, in the region of the 

 arms and again at the posterior end of the trunk. Pin back the flaps. 

 Notice the blood vessels of the skin and examine with a handlens. 



