THE TADPOLE. 



If possible the pupils should have a chance to examine 

 tadpoles of different ages. These can readily be obtained 

 by collecting the eggs in the spring and allowing them to 

 hatch out in glass jars. A number of these can be killed 

 at various stages by means of picrosulphnric acid (see Ap- 

 pendix) used for a couple of hours, then washed two to three 

 hours in water, and preserved in 70$ alcohol. The earliest 

 stage necessary should show the external gills, the latest 

 should have the hind legs well formed. 



In the earliest of these larvae the pupil should pay especial 

 attention to the gills; the tail with its fin, how does it 

 differ from that of fishes ? In the older larvae the jaws 

 should be examined. What is their nature ? What is the 

 size of the mouth compared with that of the adult ? On 

 the left side of the body see the opening of gill-chamber. Is 

 there one on the right side ? Carefully open this chamber. 

 Do the right and left sides of the gill-cavity connect ? Can 

 you find any traces of the fore limb ? Carefully open the 

 abdomen and notice the compact coiling of the intestine. 

 Is it relatively longer or shorter than in the adult ? Pick 

 away the muscles from one side of the body until the middle 

 line of the body is reached. Do you find any vertebrae ? 

 Lying in this median line find a continuous gelatinous cord, 

 the notochord. 



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