GROWTH OF AMPHIBIA AFTER THYROIDECTOMY 31 



less than that of thyroidless larvae of the same age and, as shown 

 in our material, the relative volumes of the thymus of the two 

 groups of animals are about the same. The variability of the 

 thymus is so great, as shown by our own and Rogers' material, 

 that it cannot be stated with certainty that the thymus is rel- 

 atively larger in young frogs than in the larger thyroidless 

 larvae, although probably it is so. It is relatively larger, how- 

 ever, in frogs than in thyroidless larvae which are the same size 

 as the tadpoles from which the frogs have been transformed. 

 In our 1918 series some of our thyroidless larvae attained nearly 

 their maximum possible size in June and July, but their thymus 

 glands were not relatively smaller than in those 1917 larvae 

 which reached this size in the autumn or winter, so the age re- 

 lationship emphasized by Rogers on page 597 does not exist. 

 Rogers' graph on page 599 is very misleading. It indicates that 

 normal tadpoles have thymus glands several times as large as 

 those of the thyroidless larvae of the same age, whereas such 

 is not the case. The difference in the two curves at this point 

 is due to the use of the single small thymus of the thyroidless 

 larva referred to above and does not represent the true condition. 

 Both these graphs and those on page 598 are based upon few 

 incorrectly measured volumes incorrectly standardized. 



Three of our thyroidless larvae which survived the second 

 season were autopsied and of these, two had thymus glands 

 that were relatively somewhat smaller than the general average 

 of the group of younger large thyroidless larvae, but the thymus 

 of the third larva was fully as large as that of any of the younger 

 specimens. This shows that the thymus will continue to remain 

 large in some of these animals, at least, that are kept in the larval 

 condition for more than one year. Frogs at all ages have actu- 

 ally a smaller thymus than the large thyroidless larvae and the 

 large growth of the gland in the latter might be considered as 

 an hypertrophied condition, but it must be admitted, as in the 

 case of the other organs of these larvae, that there is no standard 

 of comparison ' for them. If the control tadpoles could be in- 

 duced to grow as large as these experimental larvae, their thymi 

 might also become large. Figures 47 and 49 show, however, 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 29, NO. 1 



