Introduction 



ment which is wholly dependent on temperature is materially 

 aided. Each egg is surrounded by a thin elastic membrane that 

 fits it closely (except in Pelobatidse) and one or two gelatinous 

 membranes outside of this. These membranes are not visible 

 when the eggs are first laid, but swell after they come into con- 

 tact with water, becoming very conspicuous, though transparent. 

 These envelopes aid the development ; they absorb and hold the 

 warmth from the sun's rays, producing a somewhat higher and 

 more equable temperature than is that of the surrounding water. 

 They also serve to protect the egg or egg mass by converting it 

 into a slippery unmanageable object not easily grasped and eaten 

 by birds, turtles, and others of the Batrachian's enemies about 

 the pond. It is reported that ducks sometimes feed upon frog 

 egg masses. The membranes of the eggs are easily penetrated 

 by leeches,' which suck out the contents, and so prove themselves 

 perhaps the most destructive enemy of frog spawn. 



The breeding season is likely to occur in early spring, but 

 varies with the species, some of the Salientia being able to en- 

 dure more cold than others. As a rule, toads are later in their 

 appearance at the pond than are frogs or tree frogs. In eastern 

 North America, Rana sylvatica and Rana -pipiens appear earliest 

 among the frogs, Hyla pickeringii and Chorophilus n. feriarunt 

 among the tree frogs. The latter may be heard as early as 

 February and the two species of frogs appear very soon after. 

 On the Western coast, Rana draytonii; and Hyla regilla are 

 breeding in January and February in California, and following 

 them closely Rana preiiosa and Hyla regilla are breeding in Wash- 

 ington. Members of the genus Scaphiopus have been known to 

 breed at various times from March to August, always during a 

 warm rain. 



The length of the breeding season varies greatly. All of a 

 given species may resort to the water at about the same time 

 and remain there for a short period only, as in the case of the 

 Spadefoot Toad (Scaphiopus holbrookii) or less conspicuously of 

 the Wood Frog (Rana sylvatica). On the other hand, they 

 may go to the ponds scatteringly and each individual linger there. 

 In the latter case the breeding season may extend over a period 

 of two or three months, as in the case of Bujo leniiginosus, of 

 Bufo americanus, and even more remarkably of Bufo fowleri. 



^Macrobdetta decora. 



