SECTION I. EXTERNAL CHARACTERS 



Place the frog on your dissecting pan, dorsal side up, and 

 locate the following structures: 



1. The head is roughly triangular in shape. 



2. The mouth is large, terminal, and probably tightly 

 closed. 



3. The eyes are large and bulging. In the living speci- 

 men a central pupil, black and oval, surrounded by 

 a bright, colored iris, can be distinguished. 



4. There are two eyelids, upper and lower (the latter 

 with an attached nictitating membrane which can 

 with difficulty be demonstrated). 



5. The brow spot, a light spot between the eyes, is the 

 vestige of the pineal eye, and embryologically con- 

 nected with the epiphysis (176). 



6. Caudal to the eyes is the tympanic membrane 

 (tympanum) of the ear, in the center of which is a 

 slight projection caused by the columella (165), one 

 of the ear bones. What significance do you find in 

 its position on the tympanum? 



7. At the cephalic end, the external (anterior) nares, 

 or nostrils, are found. 



8. The trunk bears two pairs of appendages. 



9. The fore limb consists of upper arm, lower (fore) 

 arm, and hand; the latter has four fingers and a 

 vestigial thumb. The first and second fingers have 

 three joints each, the others four. 



10. The hind limb consists of thigh, shank, and foot; 

 the latter has five toes and a rudimentary sixth 



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