THE ANIMAL CLIMAX-THE CHORDATES 



299 



Fig. 13-23. The common leopard frog (Rana pipiens). 



is the leopard frog, Rana pipiens (Fig. 

 13-23), found in shallow ponds and 

 streams. The small, green tree toad, Hyla 

 versicolor (Fig. 13-24), emits its musical 

 sounds from wooded areas near ponds, 

 while the bullfrog, Rana cafesbiana (Fig. 

 13-25), gives forth its resounding bellows 

 from larger bodies of water. While these 

 "delightful" sounds are very much wel- 

 comed by everyone, they have more impor- 

 tant meaning to the frogs. The male frogs 

 usually emerge from hibernation first and 

 begin the croaking in order to attract the 

 females who follow some days later. At this 

 time of the year, the eggs in the body of 

 the female are fully mature; as a matter of 

 fact, they have been fully ripe for many 

 months, waiting the coming of the breeding 

 season. The male mounts and clasps the 



female with his front legs, grasping her just 

 back of her front legs and pressing the 

 small swollen parts of each thumb ( nuptial 

 pads) against her breast. This process is 

 called amplexus (Fig. 13-26), and is a kind 

 of copulation. As the female lays her eggs 

 the male discharges a milky fluid containing 

 sperms, thus fertilizing them. Very shortly 

 thereafter, the gelatinous matrix surround- 

 ing the eggs swells, causing them to adhere 

 to twigs or any other underwater debris 

 (Fig. 13-27). The egg mass resembles 

 cooked tapioca, and the eggs themselves are 

 at first black above and white below, a 

 characteristic which may possibly offer 

 some degree of protection during the early 

 stages of development. 



If an egg is viewed under the microscope, 

 one can see it divide, once, twice, three 



