THE FROG, A TYPICAL VERTEBRATE 267 



their history, and since the layers are the germs which give rise 

 to the organs of the body, they are called germ layers. 



This embryo moves about within the egg by means of cilia, 

 but these soon disappear after hatching. The tadpole, after 

 breaking out of the egg, lives for a few days on the yolk in its 

 alimentary canal, and then feeds on algae and other vegetable 

 matter. The external gills grow out into long, branching tufts 

 (Fig. 153, 20). Four pairs of internal gills are formed later, and 

 when the external gills disappear, these function in their stead, 

 the water entering the mouth, passing through the gill slits, and 

 out of an opening on the left side of the body, called the spiracle. 



The hind limbs appear first (Fig. 153, 5). Later the fore limbs 

 break out (6) . The tail decreases in size as the end of the larval 

 period approaches (7) and is gradually resorbed. The gills are 

 likewise resorbed, and the lungs develop to take their place as 

 respiratory organs. Finally the form resembling that of the 

 adult frog is acquired (8). 



REFERENCES 



The Biology of the Frog, by S. J. Holmes. — The Macmillan Co., N. Y. City. 

 The Frog Book, by M. C. Dickerson. — Doubleday, Page and Co., N. Y. 

 City. 



