244 A STUDY OF VERTEBRATES 



containing much and some containing little oxygen. Before the blood 

 from the tissues and lungs has time to mix, however, it leaves the ventricle 

 and by a deUcate adjustment in the vessels leaving the heart most of the 

 blood containing much oxygen is passed to all the various organs of the 

 body, while the blood deficient in oxygen, but containing a large amount 

 of carbon dioxide, is pumped to the lungs, where an exchange of oxygen 

 and carbon dioxide takes place by osmosis. 



In the tissues of the body wherever work is done the process of burning 

 or oxidation must take place, for by such means only is the energy neces- 

 sary to do the work released. Food in the blood is taken to the muscle 

 cells or other cells of the body and there oxidized. The products of the 

 burning — carbon dioxide — and any other organic wastes given off from 

 the tissues must be eliminated from the body. As we know, the carbon 

 dioxide passes off through the lungs and to some extent through the skin 

 of the frog, while the nitrogenous wastes, poisons which must be taken 

 from the blood, are eliminated from it in the kidneys. 



Change of Form in Development of the Frog. — Not all verte- 

 brates develop directly into an adult. The frog, for example, 

 changes its form completely before it becomes an adult. This 

 change in form is known as a metamorphosis. Let us examine 

 the development of the common leopard frog. 



The eggs of this frog are laid in shallow water in the early 

 spring. Masses of several hundred, which may be found at- 

 tached to twigs or other supports under water, are deposited at 

 a single laying. Immediately before leaving the body of the 

 female they receive a coating of jelly like material, which swells 

 up after the eggs are laid. Thus they are protected from the 

 attack of fish or other animals which might use them as food. 

 The upper side of the egg is dark, the light-colored side being 

 weighted down with a supply of yolk (food). The fertilized egg 

 soon segments (divides into many cells), and in a few days, if the 

 weather is warm, these eggs have each grown into an oblong body 

 which shows the form of a tadpole. Shortly after the tadpole 

 wriggles out of the jellylike case and begins life outside the egg. At 

 first it remains attached to some water weed by means of a pair 

 of suckerlike projections; later a mouth is formed, and the tad- 

 pole begins to feed upon algae or other tiny water plants. At 

 this time, about two weeks after the eggs were laid, gills are 



