116 ANIMAL BIOLOGY 



float on the surface of the water 1 (Fig. 84, B). Each fertil- 

 ized egg is a small sphere, black on its upper surface and white 

 beneath, and inclosed in a gelatinous covering. The 

 warmth of the sun causes the one-celled egg to divide verti- 

 cally in half to form the two-celled stage (Fig. 85, B) and the 

 process of division continues until the egg consists of many 

 cells (Fig. 85, H). Food for the development of the embryo 

 is stored in the egg. 



The many cells of the egg. gradually become different in 

 character .and so form the various organs of the embryo 



(Fig. 86) . Soon after hatching, 

 the young of the frog, known as 

 tadpoles, secure their food by 

 sucking in tiny -water plants 

 found on the surface of plants 



FIG. 86. Embryo of frog. , /T ^. O A T\\ m j 



and stones (Fig. 84, D). Tad- 

 poles resemble fishes in having gills for breathing, a heart 

 with two chambers instead of three, and a tail for locomotion. 

 At first the gills are on the outside of the body (Fig. 84, E), but 

 later four pairs of internal gills are formed, and the external 

 gills are absorbed. The animal increases in size, the hind legs 

 appear, and the arms are formed beneath the skin. Meanwhile 

 the lungs are being developed, the heart becomes three cham- 

 bered, the legs grow larger, arms appear, and finally the 

 gills and the tail are completely absorbed. The tadpole 

 now leaves the water, since it is an air-breathing animal. 

 This succession of changes after hatching from the egg is 

 known as a metamorphosis. 



87. Relatives of the frog. Much like the frog in structure and 

 life history is the common garden toad. Toads, however, in their 



1 If possible eggs in different stages of segmentation should be se- 

 cured, preserved in 5 per cent formalin, and used for demonstration. 



