262 



AMPHIBIANS. 



extruded from the female. As a rule, these eggs, which much resemble those of 

 fish, are small, very numerous, and connected together by mucilage, forming either 

 a string or a jelly-like mass in which the dark yolks are very conspicuous. Some 

 of the tree-frogs, however, lay large eggs, within which the larvae undergo, the 

 whole of such transformation as takes place ; and in one genus, instead of the 

 usual gills, a temporary breathing-organ is developed on the tail. A land-frog in 

 the Solomon Islands also lays large eggs, like small marbles, which are deposited 

 in the crevices of rocks, and from which emerge fully-developed frogs. The eggs, 

 with certain exceptions, are deposited in water, where they are hatched by 

 the heat of the sun ; and it appears that the dark colour of the yolk is for 

 the purpose of absorbing as much solar heat as possible. Such eggs as are 

 laid during the late spring and summer are less darkly coloured, and have 

 thinner coats, than those deposited in the early part of the spring ; and while the 



DEVELOPMENT OK Till: FHOG. 



1, Eggs when first laid ; 2, Eggs at a later stage ; 3, Egg containing embryo ; 4, Newly-hatched larvte ; 

 5, 6, Larva; with external gills ; 7-12, Later stages in the development of larvae. 



former are placed on the ground at the bottom of the water, the latter float on the 

 surface ; the reason of this difference being that in the early part of the year 

 the lower strata of water are too cold to admit of the development of the ova. 

 In ordinary cases, when the larva has reached a certain stage, it bursts the 

 investing membranes of the egg, and comes into the world adapted for an 

 aquatic life, and always possessing a long compressed tail composed of zigzag- 

 shaped masses of muscles, similar to those of fishes. The first process jg the 

 sprouting forth of branching external gills from the sides of the neck, which in 

 the larvae of the frogs and toads are subsequently replaced by internal gills, but in 

 the long-tailed forms persist for a longer period. After the disappearance of the 

 external gills, the water is expelled from the gill-chamber by one or two tubes, 

 generally discharging by a single orifice, which may be situated either on the 

 lower surface of the body, or on the left side. As soon as the external gills have 

 made their appearance, development is concentrated on the tail and the absorption 



