FROGS AND THEIR RELATIVES 107 



the forceps ; carefully insert the point of the scissors 

 near the posterior end of the trunk, and carefully cut 

 forward on one side of the body as far as the tip of 

 the head, and back on the other side of the trunk, 

 until the skin is completely removed from the ventral 

 surface. In a similar manner remove the muscular 

 wall that covers the trunk, being careful not to injure 

 the internal organs. If time allows, remove also the 

 skin from one leg ; call attention to the thinness of 

 the skin and to the underlying blood vessels ; show the 

 characteristics and action of the leg muscles. 



If the specimen is a female, remove nearly all the eggs 

 and throw them away. Insert a blowpipe in the glot- 

 tis and partly inflate the lungs. Wash the specimens 

 thoroughly to remove all traces of blood and cover 

 them with water in a dissecting pan. 



If the specimens are needed on successive days, they 

 should be wrapped in a wet cloth immediately after 

 the class work of each day and kept in a cold place. 

 Use only specimens that are fresh. 



1. Make an outline drawing, natural size (or twice 



natural size if the frogs are small), of the ventral 

 view of the head and trunk regions of a dissected 

 specimen, together with the base of each of the 

 four appendages, and dfaw nothing else until 

 directed to do so. 



2. The heart is a cone-shaped body midway between the 



arms. Draw the heart to show its position, 

 shape, and relative size. 



3. On either side of the heart are the lungs. Stretch 



one of them a little by pulling on it ; then letting 



li SO. 



a. State whether or not the lungs are elastic. Are 



they hollow or solid? Of what advantage are 

 these two characteristics of structure ? 



b. What is the color of the lungs? State whether 



or not you find tiny blood vessels on the sur- 

 face. 



