EXERCISE 87 

 WHAT ARE THE STAGES IN THE DEVELOPMENT OF THE FROG'S EGG? 



Materials. Fresh eggs of the frog or toad (these may be secured in early spring, the toad's 

 eggs usually appearing a little later than those of the frog) ; large aquarium jars ; hand magnify- 

 ing glasses. 



Directions. Place the eggs of the frog or toad in an aquarium in shallow water. Observe them 

 from day to day, noting early stages of development. Some of the eggs may be examined from 

 day to day under the low power of a microscope. The following stages may be observed : 



a. If the eggs are secured very soon after they are laid it will be possible to see, with a hand 

 magnifier or under the low power of a microscope, some which have divided into 2 cells, 4 cells, 

 or a larger number of cells. 



6. At a later period the original single egg cell has divided into such a large number that the 

 individual cells of the mass cannot be separately distinguished by use of the magnifier. 



c. The mass of cells produced by the egg becomes elongated instead of spherical. This is called 

 the embryo. 



d. The embryo continues to elongate ; head and tail may be distinguished ; eyes and other 

 organs appear. 



e. The young animal frees itself from the surrounding jelly, swims through the water, and 

 usually attaches itself to floating objects or to the walls of the aquarium. At this stage it has 

 conspicuous gills (breathing organs). 



Record and describe all the changes which you have observed in the development of the egg. 



Additional problems. Can you discover how to distinguish frog's eggs from, toad's eggs ? 

 In what kinds of situations do these animals lay their eggs ? 

 At approximately what dates are frog's and toad's eggs laid in your region ? 



Newts and salamanders are animals which are related to frogs and toads in structure and habits. If you 

 can find their eggs, these animals may be grown in aquaria, as may frogs and toads. 



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