50 The Vertebrate Organ Systems 



the frog calls, it is usually lying half submerged in the water. It takes 

 in a large volume of air, then tightly closes the mouth and nostrils. 

 The air passes back and forth from the lungs to the mouth over the 

 vocal chords. Some air goes into the large vocal sacs which lie just 

 above each shoulder. The croak is repeated four or five times as the 

 sac swells and gives resonance to the croak. Suddenly the air is 

 forced out of the sac into the lungs. 



The exact manner by which the sexes recognize one another is 

 not entirely clear. Most male frogs will clasp any object which is 

 about their own size. If another male frog is clasped, it will croak and 

 often be released. The female which is unable to croak will continue 

 to be clasped. It has been suggested that this is the chief method of 

 recognition. The clasping reflex is a very strong one, and once the 

 female is clasped, it is very difficult to release the male. During clasp- 

 ing, the shorter forelegs of the male encircle the female just behind 

 her forelegs. The swollen nuptial pads on the inner digit of the 

 male's forelegs aid in maintaining this grip. The male may remain in 

 this position for several days until the eggs are extruded by the female. 

 When this occurs, the male discharges spermatozoa over them. 



Each female lays hundreds of eggs which are attached in masses 

 to grasses or twigs. It is probable that many of these are destined 

 never to hatch. Many are eaten by other animals, and some fail to 

 develop. When laid, the egg masses are quite small, about one to two 

 inches across. Soon the jelly mass surrounding the eggs absorbs water 

 and swells up to a mass of some three to five inches. 



The upper surface of each egg is dark in color while the under 

 surface is lighter. This darker upper surface probably aids in ab- 

 sorbing the heat of the somewhat chilly spring days. Development of 

 the embryo proceeds at a rate which is determined by the air and 

 water temperature. 



Before hatching, the young larvae or tadpoles can be seen moving 

 about in the gelatinous mass. When hatched, the tiny tadpole or 

 "polliwog" has an oval-shaped head and body, a slender tail, and horny 

 jaws. These jaws are useful for the scraping of tiny fragments of 

 algae from plant stems. The larvae respire by means of three pairs of 

 external gills. As growth occurs, the external gills are gradually re- 

 placed by internal gills below the gill slits. Gradually the hind legs 

 start appearing but the forelegs are still invisible below a membrane 

 on the under surface of the body. As the gills disappear and the lungs 

 begin functioning, the frog is forced to seek shallower water where 



