196 EMBRYOLOGICAL TYPES 



over the gill-slits. The external gills disappear, and so-called 

 internal gills develop in the walls of the gill-slits and subserve 

 the function of respiration. The folds just mentioned form 

 the operculum, which leaves only a small hole on the left side 

 through which the water which passes through the gill-slits 

 may escape. 



The organisation of the larva is just like that of a fish, and 

 there is little indication of the frog into which it will develop. 

 The changes which take place in the conversion of the tadpole 

 into the frog are known as metamorphosis. 



Metamorphosis. — The chief differences between the 

 organisation of the tadpole and that of the frog concerns the 

 limbs, lungs and pulmonary respiration, intestine, tongue, and 

 tail. 



The limbs arise as buds in tadpoles about half an inch 

 long, and muscles grow into them from the myotomes. The 

 buds of the forelimbs are, however, concealed beneath the 

 operculum, and are therefore invisible. Those of the hind- 

 limbs are situated at the base of the tail, on each side of the 

 cloaca. In time, the fore limbs grow out through the operculum, 

 making use of the opening on the left side and making a new 

 one on the right. Soon the limbs become visibly jointed and 

 the toes appear. 



Meanwhile, the lungs are developing, and to each of them 

 there runs a blood-vessel which is formed as a branch from the 

 efferent artery of the last or 6th arch. This vessel is the 

 rudiment of the pulmonary artery. From time to time, the 

 tadpole takes in a gulp of air at the surface of the water and 

 fills its lungs. A certain amount of oxygenation of the blood 

 now begins to take place in the lungs, and the gill-circulation 

 becomes reduced by the establishment of direct connexions 

 between the afferent and efferent branchial arteries. The gills 

 therefore become " short-circuited," and left out of the 

 circulation gradually as more and more of the blood goes to 

 the lungs to be oxygenated, and returns to the heart by the 

 pulmonary veins. The now continuous vessel in the 3rd 

 visceral arch becomes the carotid, that in the 4th becomes 

 the systemic arch, that in the 5th disappears, and the 6th as 



