102 DEVELOPMENT OF THE FROG 



each side of the anus : these steadily increase in size : about 

 the seventh week they become divided into joints ; and a week 

 later the toes appear. 



The fore-limbs arise about the same time as the hind ones, 

 but are covered by the opercular folds, and hence do not become 

 visible till a later stage. 



Towards the end of the second month the lungs come into 

 use, and the tadpoles which now have the form shown in Fig 20, 

 9 and 10, frequently come to the surface of the water to breathe. 

 The gills now begin to degenerate, but for a time respiration is 

 eflfected both by the gills and the lungs. 



A fortnight or three weeks later a distinct metamorphosis 

 occurs, whereby the tadpole becomes transformed from the fish- 

 like condition in which it has hitherto remained to the purely 

 air-breathing stage characteristic of the adult. The tadpole 

 ceases to feed ; a casting, or ecdysis, of the outer layer of the 

 skin takes place ; the gills are gradually absorbed ; the homy 

 jaws are thrown off; the large frilled lips shrink up ; the mouth 

 loses its rounded suctorial form and becomes much wider ; the 

 tongue, previously small, increases considerably in size ; the 

 eyes become larger and more prominent ; 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 intes- 

 tine 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 accompanying the change in breathing, occur in 

 the bloodvessels. 



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

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

 lengthen considerably, and the animal leaves the water as a Frog. 



Explanation of the metamorphosis. 



A tadpole is really a fish ; not merely in its habits, but in its 

 mode of breathing, in the arrangement of its heart and blood- 

 vessels, and, indeed, in almost every detail of its organisation. 



The fact that a frog should commence its life as a fish is 

 explained by the Law of Recapitulation, according to which every 

 animal is constrained during its own development to repeat, or 



