230 MANUAL OF ANIMAL BIOLOGY 



great extent by the hormone secreted by the thyroid gland. The 

 chief changes which occur during metamorphosis may now be 

 noted : 



(a) In the development of limbs, a swelling appears on either 

 side of the body at the base of the tail. These are the posterior 

 limb buds, and they continue to grow rapidly and soon form the 

 hind limbs. The fore limbs, which develop later, are formed from 

 tissues of the body wall in the gill region. If the gill covering is cut 

 away in this region at the proper time, the tiny fore limbs may be 

 found beneath ; one on either side. Before metamorphosis is com- 

 pleted the fore limbs break through the operculum, so that at this 

 stage we have a tadpole with two pairs of limbs and also with a 

 long tail. (W. f. 175, F-J.) 



(b) Shortly after the appearance of the fore limbs, the tail be- 

 gins to decrease in size, and this process continues until, by the 

 time metamorphosis is completed, it is entirely resorbed. It is not 

 clear as to just how these degenerative changes in the tissues of 

 the tail are brought about. (W. f. 175, K.) 



(c) During metamorphosis, great changes take place in the ali- 

 mentary canal. In the tadpole, this structure is very long and 

 coiled and without much differentiation between the various re- 

 gions. Such a type of alimentary tract is adapted for securing 

 nourishment from plant tissues which form the greater part of the 

 food of the tadpole. The metamorphic changes cause a great de- 

 crease in the length of the tract, and a greater degree of differenti- 

 ation between the various regions. These structural changes 

 adapt the alimentary canal for the digestion of animal tissues which 

 form the greater part of the food of the adult Frog. 



(d) During metamorphosis the lungs, which started to develop 

 very early in the embryonic life and then stopped, begin their 

 growth anew and form a pair of elongated, distensible, tubular 

 sacs with highly vascularized walls. They lie, as we know, in the 

 anterior end of the coelom and are connected by the trachea with 

 the glottis which opens in the ventral wall of the mouth. Coinci- 

 dent with the development and functioning of the lungs, the inter- 

 nal gills begin to degenerate, and the young Frog is no longer able 

 to secure oxygen from the water, but must come to the surface 

 and inhale air directly into the lungs, so that the cells and plasma 

 of the blood can carry on the respiratory interchange. These meta- 

 morphic changes having been completed, the transformed tadpole 



