LABORATORY WORK: FROG. 



EXTERNAL CHARACTERS. 



If live frogs can readily be had, the student should have 

 a chance to study them alive before dissection. Notice the 

 way in which the eyes can be retracted. Notice especially 

 the way in which the frog breathes. Watch the nostrils 

 during the operation. On the back, a little in front of the 

 vent, may be seen a pulsation. This is produced by lymph- 

 hearts beneath the skin. Kill the frogs by wrapping them 

 in a cloth moistened with chloroform, and put them in a 

 close jar for an hour. 



Notice the shape of the body. Can you find scales any- 

 where ? Is there anything like a tail ? How many appen- 

 dages are there ? How do they compare with your own 

 limbs? Open the mouth; where do you find teeth? 

 Where are the nostrils ? Probe them with a bristle. Where 

 does this appear in the mouth ? How does the tongue 

 differ from your own ? 



Behind and a little below the eye is a circular tympanic 

 membrane (connected with the auditory apparatus). Cut 

 through this and insert a probe. Where does this appear 

 in the mouth ? With what does this Eustachian tube most 

 nearly correspond in the shark ? See the way the mouth- 

 cavity narrows behind to form the gullet. In front of this 

 see the slit-like glottis in the floor of the mouth. 



In the fore limbs do you find parts corresponding to 

 arm, forearm, wrist, palm, and fingers ? How many fingers ? 



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