METAMORPHOSIS OF NEOTENOUS AMPHIBIANS 411 



33.5 mm., without limbs, brings about very marked metamorphic 

 changes within thirty days, although the animals could not be 

 reared to the stage of complete tail resorption. 



It was observed that in the engrafted animals autolysis of the 

 skin over the region of the fore limbs occurs independently of 

 limb development as a distinct phenomenon of anuran meta- 

 morphosis. Years ago Braus ('06) described similar phenomena 

 in developing tadpoles after extirpation of the limb bud. I men- 

 tion it here because of the remarkable autolysis which is sometimes 

 observed in transplanted larvae; the fore limbs may be small, 

 whereas the skin area destroyed may be very large indeed com- 

 pared with limb size. It should be remembered, however, that 

 in anurans the fore and hind legs tend to keep pace with one 

 another in development, only the fore legs are not visible because 

 of the opercular covering. 



The chief point of interest, however, is the odd fact that much 

 greater metamorphic change follows transplantation of the thy- 

 roids of 80-mm. neotenous larvae with hind legs 11.6 mm. into 

 immature larvae without limbs than occurs in control animals 

 80 mm. with legs 11.5 mm. In other words, the grafted glands 

 wrought far greater changes in the same time interval (approx- 

 imately one month) when transplanted into immature larvae 

 than when left undisturbed in the mature forms. 



This result, so curious and at variance with what one might 

 expect, led to a repetition of the same experiment the following 

 summer, from a different angle. Immature larvae averaging 40 

 mm. total length with hind leg buds 0.5 mm. were transplanted 

 with the thyroids of mature though not neotenous tadpoles 

 68 mm. total length with hind legs 11.5 mm. The results were 

 similar to those of the earlier experiments, though not so striking 

 as one would expect, because the mature control animals (not 

 neotenous) in this experiment were approaching metamorphosis 

 at the end of the experiment, w^hereas in the previous experiment 

 the neotenous controls showed no change, and in fact passed the 

 winter in the laboratory as tadpoles. 



The experiments indicate that the thyroids of extra-season, 

 neotenous anuran larvae with hind limbs 11 mm. long are physio- 



THB JOURXAL OF EXPERIMENTAL ZOOLOGY, VOL. 36, NO. 4 



