176 VERTEBRATES 



almost any pond, and may be identified as small dark bodies 

 floating in a jellylike mass. They should be put in a shal- 

 low basin, in a warm room with plenty of sunlight in it, and 

 to insure growth the water should be changed every day. 



If everything goes well the student may study the changes 

 that take place from the day the eggs are collected. -By 

 putting the eggs under a dissecting microscope or the low 

 power of the compound microscope, the student may see the 

 segmentation of the yolk, the expanding and extending of 

 the mass and finally the appearance of the head and tail of 

 the young frog, or tadpole. The animal now moves about 

 freely, breathes by external gills, and grows rapidly. 



Study a specimen every day until the external gills give 

 way and the internal gills are used. Find how the water 

 reaches the internal gills. Study the structure of the gills 

 under the microscope and make drawings. 



Note the appearance of the hind legs. Study their form. 

 Are they perfect when they first appear? Draw the animal 

 in this stage. Dissect to see the structure of the alimentary 

 canal and heart, and the relation of the gills to the heart. 



When the front legs appear the gill opening closes. 

 Where do the front legs appear? What takes place then? 

 Do the young frogs spend all the time in water now? Do 

 they spend much of their time on land? What is the food 

 of a tadpole? Of a frog? Dissect the tadpole in which 

 the front legs have appeared. Do you find any change in 

 the digestive system ? Study the structure of the lungs. Do 

 they seem to be related closely to the alimentary tract? 

 Do they seem to be folds in the pharynx? Draw. What 

 advantages does this change from water life to land life 

 bring? What disadvantages? 



