102 DEVELOPMENT OF THE FROG 



the fore-limbs appear, the left one being pushed through the 

 spout-like opening of the branchial chamber, and the right 

 one forcing its way through the opercular fold, in which it 

 leaves a ragged hole. The abdomen shrinks ; the stomach 

 and liver enlarge, but the intestine becomes considerably 

 shorter than before, and of smaller diameter ; the animal, 

 previously a vegetable feeder, now becomes carnivorous. 

 The gill-clefts close up ; and important modifications accom- 

 panying the change in breathing, occur in the blood- 

 vessels. 



The tail, which is still of great length, Fig. 20, g , now 

 begins to shorten, and is soon completely absorbed ; the hind- 

 limbs lengthen considerably, and the animal leaves the water 

 as a Frog, Fig. 20. 



Explanation of the Metamorphosis. 



The structure and mode of life of the tadpole (opercular 

 gills ; gill slits ; a tail ; and lateral sense organs) give it a 

 fish-like character. But there is no known fish which at 

 all closely resembles the tadpole. The mud-fish of Africa 

 and America approach it. Still more close is the agreement 

 between the tadpole and the young stage of these fish. 

 Hence we may say that the most probable explanation of 

 the problem, " Why the frog first becomes a tadpole ? " is 

 that its fish-like ancestors passed through a similar stage 

 in their career. The frog, in brief, climbs up its genealogical 

 tree in its own life-history. 



Simplification of the Frog's Structure. 



Before following the development of the frog in detail it 

 will be useful if we imagine its structure to be reduced to 

 the most essential features ; and anticipating one or two of 

 the developmental steps, trace in thought the simplification 

 of these essential organs to the earlier stages and ultimately 

 to the egg. 



From this point of view we may look upon the frog as 

 simply a trunk, disregarding the head and limbs. This step 

 is justified by the structure of the lowest vertebrates in 



