CH. VIIl] FROGS, TOADS, AND NEWTS 285 



colour has spread over the whole egg. By the end of 

 the first week after fertilisation the shape is no longer 

 spherical but decidedly ovoid, the embryo becoming 

 much longer than broad. By the tenth and eleventh 

 day head, body, and tail are recognisable. The tail 

 grows rapidly, and from the sides of the throat, two and 

 then a third pair of external gills project as branching 

 filaments. The entire growth thus far takes place at the 

 expense of the food-yolk contained within the egg at the 

 time of deposition. About a fortnight after fertilisation 

 of the eggs the young tadpoles escape from the gelatinous 

 mucous envelopes and swim in the water by means of 

 their well-developed tail. They are now from a third 

 to half an inch long. The whole surface of the body is 



Fig. 49. 



1. Tadpoles soon after hatching, clinging to water-weeds. 2. Tadpole 

 with two pairs of external gills. 3 and 4. Tadpoles with operculum 

 forward and forming. 5. Tadpole with well-developed hind legs. 



