DEVELOPMENT OF THE FERTILIZED EGG 289 



The tail also shortens until it disappears. It does not drop off, 

 but is gradually absorbed into the blood vessels and carried 

 to the rest of the body, where it is used as nourishment for the 

 other parts of the body. These changes are not abrupt but take 

 place gradually as the animal assumes the adult form; Fig. 

 133, F to K. 



By the time the form shown in Figure / is reached, the gill 

 slits have entirely closed, the skin growing over them; and from 

 this time on the animal takes air into its mouth and forces it 

 into its lungs in the ordinary fashion of the adult frog. It 

 changes, therefore, from a water-breathing to an air-breathing 

 animal. But even when it is an adult, the animal never quite 

 loses its power of respiring by means of water, for the skin 

 of the adult frog is always kept moist, and contains abundant 

 blood vessels by means of which oxygen can be absorbed from 

 the water, and carbon dioxid excreted. Not until the gill slits 

 have closed and the lungs have become functional is the frog 

 able to leave the water and live in the air. By this time its 

 legs have become well grown and are strong enough to enable 

 it to move more or less vigorously on the land, so that the 

 tadpole may leave the water and assume its adult habits. 



Other Types of Metamorphosis. Such a series of changes 

 from the embryo to the adult is known as metamorphosis. 

 Many other animals besides the frog have a metamorphosis. 

 One of the best-known examples is the metamorphosis of a 

 butterfly, which hatches as a caterpillar, lives a considerable 

 part of its life in this stage, and then passes into a pupa inside 

 of a cocoon. Here it remains dormant for a considerable time 

 and eventually emerges in the form of a winged butterfly, the 

 imago. Many other types of metamorphosis are found among 

 animals, for it is quite common for them to pass through a 

 series of stages in their development, each stage being different 

 from the other, and each different from the adult. Not all 

 animals, however, have a metamorphosis, many passing by a 

 very direct course to the adult stage. In the ordinary chick, 



