Development 



may remain aquatic or may become terrestrial in habit. (Figs, i, 

 2, and 3.) The aquatic forms usually have the tail flattened 

 vertically to aid in swimming; while the terrestrial forms have 

 a rounded tail. A few, like Necturus, keep the gills throughout 

 their lives. 



The Urodela represent in numbers about one-tenth of the 

 Batrachian group as known in the world at present, that is, about 

 one hundred species. Of this number between sixty and seventy 

 are American species. 



III. Development and Metamorphosis 



The North American representatives of the Salientia deposit 

 the eggs in water, usually in shallow, stagnant water. The eggs 

 may be laid singly or in small clusters, as in the case of some 

 of the tree frogs (Fig. 167); in large masses, as is the habit among 

 the frogs (Fig. 246) ; or in long unbroken strings, as in the case of 

 toads (Fig. 34). The eggs are sometimes free in the water, but 

 more often are attached to water-weeds or other objects. If free, 

 the egg mass has such buoyancy that it floats at the surface of 

 the water (Rana sylvatica). 



The number of eggs in a laying depends on the species and 

 often on the age and size of the individual of the species. The 

 number may be as high as 12,000 in the American Toad. The 

 size of the egg will sometimes help in the identification of a 

 species, but there is great variation in this respect. In the first 

 place, the size of the egg does not correspond with the size of the 

 Batrachian, the largest frog of North America {Rana cateshiana) 

 having an egg of less size than that of some of its smaller relatives. 

 Besides, the size varies considerably with the individual. In a 

 large collection of eg^ masses of Rana sylvatica or Rana palustrts, 

 the difference in size will be so conspicuous that unless one 

 knew the opposite to be true, he would judge the egg masses to 

 have been laid by frogs of different species. 



The egg is spherical, and is provided with a large amount of 

 light-coloured yolk. When the eggs are first laid, part of this 

 yolk can be seen occupying the lower portion of the egg under 

 the more or less pigmented upper portion. (Fig. 202.) The dark 

 pigmented portion of the egg at the top (later the whole surface 

 becomes black or brown in colour) allows a greater absorption 

 of the sun's heat than would a lighter colour. So the develop- 



3 



