506 



BENNET M. ALLEN 



It will be noted that the thyroidless tadpoles retain the great 

 length of the intestine characteristic of larval life. A compari- 

 son of the total length of stomach and intestines of the thyroid- 

 less tadpoles shows a large amount of variability that cannot be 

 readily explained. Compare the cases of specimens killed Feb- 

 ruary 7, 14 and 22. A careful study of sections of the thyroid 

 region of these specimens failed to reveal the slightest vestige of 

 a thyroid gland in any case. While the size of the February 7 

 specimen may partly explain the greater intestinal length this 

 factor does not explain the difference in its length in the February 

 14 and 22 specimens. 



The experiment was made of feeding a thyroid preparation to 

 two of the thyroidless tadpoles. This was administered in the 

 form of dried flour paste in which powdered thyroid had been 

 mixed in the proportion of 1 part of thyroid to 5 of flour. The 

 results are given in table 2 : 



TABLE 2 

 Length of Limbs 



Control, thyroidless 



Thyroid fed, thyroidless 



Control, thyroidless 



Thyroid fed, thyroidless 



3 < 



mm. 

 237 

 143 

 190 

 68 



It will thus be seen that tadpoles deprived of the thyroid 

 gland may be retarded in their development for over four months 

 and will then respond to thyroid feeding to at least a certain ex- 

 tent when this is undertaken. This feeding results in a general 

 tendency toward metamorphosis evident in the changed form of 

 the head, the lengthening of the legs and the marked shortening 

 of the alimentary tract. Enough has been accomplished to show 

 that administration of thyroid extracts to a thyroidless tadpole 

 does cause it to resume the development toward metamorphosis 

 that had ceased as a result of the absence of the thyroid gland. 



