CONCERNING TADPOLES i8i 



of the life-history of the frog, for instance, we may recognise 

 four stages — the embryonic, the larval, the tadpole, and 

 the adult — and this transformation of the larva into the 

 tadpole, and finally into the adult stage forms what is 

 technically called a " metamorphosis." 



Let us follow these changes as they are unfolded in 

 the common frog, for example. The eggs, as everybody 

 knows, are deposited in large masses in ponds and ditches. 

 Such masses are commonly referred to as " spawn." When 

 first set free into the water these eggs look like little black 

 beads, surrounded by a thin transparent layer of jelly : 

 but speedily the jelly absorbs a large amount of water 

 and swells up so that the black central portion, which con- 

 tains the germ of the frog that is to be, and the food yolk 

 on which it is to live, lies in the centre of a crystal globe. 

 This outer envelope serves to protect the young embryo 

 from pressure, and also from predatory insects, while the 

 black layer of pigment surrounding the egg itself serves 

 to attract the warmth of the sun's rays necessary for 

 development. If a number of hen's eggs were broken into 

 a basin, care being taken not to rupture the yolks, a mass 

 would be produced similar to frog's spawn : the yeUow 

 yolks answering to the frog's eggs, and the transparent 

 " whites " to the gelatinous envelopes of the spawn. And 

 just as the chicken is formed neither from the yolk nor the 

 white of the hen's egg, but from a minute germ, so also is 

 the frog developed from a similar tiny germ. The black 

 ensheathed yolk and the swollen, gelatinous outer covering 

 of the frog's egg, and the yellow yolk and transparent white 

 of the hen's egg, form the food with which these germs, 

 indistinguishable so far as chemical analysis and micro- 

 scopic examination go, slowly build up in the one case a 

 bird, in the other a frog. The formation of creatures so 



