METAMORPHOSIS IN FROG LARVAE 533 



feeding experiments with Rana pipiens, thyroid feeding greatly 

 accelerated metamorphic changes, it had no effect upon the 

 gonads and germ cells. The manner in which the body changes 

 occurred in the thyroid-fed larvae of the unknown species was 

 identical with those observed for Rana pipiens, and seems to 

 be characteristic reaction of frog larvae to thyroid feeding. 



August 4, 1915, a number of larvae of the common bull frog, 

 Rana catesbiana, were brought to the laboratory. The animals 

 averaged 35 mm. in length. None of the animals revealed any 

 indications of limb buds. Thirty-five of the larvae were selected 

 for thyroid feeding, with a similar number of controls. The 

 method of administering the extract was the same as that de- 

 scribed for the previous experiments, the larvae of both experi- 

 mental and control groups were kept in the sunlight as much as 

 possible. 



August 14, ten days from the beginning of the experiment, 

 metamorphic changes characteristic of hyperthyroidism ap- 

 peared. The bodies of the larvae were somewhat emaciated, 

 the heads elongated and all of the animals had limb buds. 



By August 18, these metamorphic changes had become very 

 pronounced. The tadpole mouth was transforming into that 

 characteristic of frogs; the tails of the larvae were considerably 

 atrophied; and the fore limb of several animals had appeared. 

 The animals died a few days later. 



None of the changes just described were observed in the 

 animals of the control group. 



When one considers that it requires two and sometimes three 

 seasons for a Rana catesbiana tadpole 35 mm. long to develop 

 to the period of complete metamorphosis, the extreme rapidity 

 with which metamorphic changes were brought on by the thyroid 

 stimulus is startling. The intense sunlight undoubtedly acted 

 in accelerating the effect of the thyroid, for I have never been 

 able to induce metamorphic changes so rapidly since. 



When the germ glands of the thyroid-fed and control larvae 

 were examined macroscopically, no differences worth recording 

 were observed. The glands of the animals of both groups varied 

 from one another somewhat in size. Taking the groups as a 



