GENERAL SURVEY OF THE FROG 3 



ear, which covers the cavity of the middle ear. The external ear and the passage 

 leading in from it, prominent in ourselves, are entirely wanting in the frog. Near 

 the center of the drum membrane, a small projection may usually be noticed; 

 this is the end of the columella, a small bone which transmits inwardly the vibra- 

 tions of the tympanic membrane. 



In the median line of the head, slightly anterior to the level of the eyes, 

 a small light-colored spot, the brow spot, may usually be found. In dark indi- 

 viduals, however, it may be concealed by pigment. In the embryonic develop- 

 ment of the frog, this spot is in connection with a portion of the brain, called 

 the pineal body, and it itself is the useless vestige of a former third medially 

 located eye (Holmes, p. 64). 



On the dorsal side of the trunk extending posteriorly from the eyes note two 

 light-colored ridges, where the skin is much thickened owing to the presence of 

 large poison glands underneath. These ridges are called the dorsolateral dermal 

 plicae. At the posterior end of the trunk on the dorsal side between the bases 

 of the hind legs is a small opening, the anus, which is the end of the digestive 

 tract. In the middle of the back a characteristic hump is present, owing to an 

 alteration at this place in the structure of the vertebral column (see a dried 

 skeleton, or consult Holmes, Fig. 63, p. 230). 



The fore limb is short and consists of three divisions, upper arm, forearm, 

 and manus or hand. How many fingers has the hand ? To which of your fingers 

 do these correspond (Holmes, p. 65) ? The rudiment of the missing finger 

 may be felt under the skin on the inner side of the hand at the base of the present 

 first finger, and may be seen on the skeleton of the hand (A). In the male 

 frog, the inner finger is enlarged and swollen at the base, especially during the 

 breeding season. 



The hind limb likewise consists of three parts, thigh, shank, and pes, or foot. 

 The ankle is remarkably elongated. There are five toes and a rudiment of sixth, 

 called the prehallux, may be felt on the inner side of the smallest toe, which cor- 

 responds to our great toe. Such additional fingers occur not infrequently among 

 the vertebrates, but their morphological significance is unknown. 



The skin is smooth, slimy in life, and possesses neither hairs, scales, nor claws. 

 Note that in general it is more loosely attached to the body than in most animals. 



Make an accurate drawing of the frog from the dorsal side, putting in all of 

 the structures and parts which have been mentioned. Before beginning to 

 draw re-read carefully the directions about drawings. Label this and all sub- 

 sequent drawings fully. 



NOTE. At the close of the first laboratory period, make an incision about one- 

 half inch long through the skin only of the left abdominal wall of your frog and 

 place the animal in the jar of preserving fluid which will be assigned to you. 

 The animal must always be kept in this jar when not in use. It must never be 

 left out on the tables, and never allowed to become dry. 



