288 THE VERTEBRATE ANIMALS 



tains considerable carbon dioxide ; the blood entering the left auricle comes 

 from the lungs, hence it contains a considerable amount of oxygen. Blood 

 leaves the heart through the Aentricle, which thus pumps blood contain- 

 ing much and little oxygen. Before the blood from the tissues and lungs 

 has time to mix, however, it leaves the ventricle and by a dehcate adjust- 

 ment 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 necessary 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 prod- 

 ucts 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. 

 Thus wastes are passed off from the body. 



Frohlem XXXVIII. The development of a frog. (^Laboratory 

 MamiaJ, Proh. XX.YVI.) 

 (a) Conditions favorable. 

 Of) Metamorphosis. 

 (o) Development of a toad (.optional). 



Field and Home Work. — During the first warm days in March or 

 April, look for gelatinous masses of frogs' eggs attached to sticks or water 

 weed in shallow ponds. CoUect some and try to hatch them out in a 

 shallow dish in the window at home. Make experiments to learn whether 

 temperpjture affects the development of the egg in any way. Place eggs 

 in dishes of water in a warm room and in a cold room, also some in the 

 ice box. Make observations for several weeks as to rate of development 

 of each lot of eggs. Also try placing a large number of eggs in one dish, 

 thus cutting down the supply of available oxygen, and in another dish 

 near by, under the same conditions of light and heat, place a few eggs. 

 Do both batches of eggs develop with the same rapidity? In all these 

 experiments be sure to use eggs from the same egg mass, so as to make sure 

 that all are of the same age. 



Development. — The eggs of the leopard frog are laid in shallow 

 water in the early spring. Masses of several hundred, which may 

 be found attached to twigs or other supports under water, are de- 



