GROWTH OF AMPHIBIA AFTER THYROIDECTOMY 35 



mm., at about the time when the corresponding thyroidless larvae 

 begin to grow more rapidly than the controls. The two phenomena 

 may, however, be only concomitantly and not causally related, 

 and the rapid growth of the thjToidless larvae may be due to 

 the hypertrophy of the hypophysis or to some other cause, and 

 not to the absence of thyroid secretion. 



The thyroid consists of large follicles filled with colloid and 

 lined by cuboidal epithelium with large round nuclei and granular 

 cytoplasm (figs. 73 and 74). There is a capsule, but very little 

 interfoUicular tissue. During metamorphosis there is no change 

 in the structure of the thyroid, but there occurs a large increase 

 in relative size. 



Figure 70 shows a section of a regenerated thyroid of a meta- 

 morphosed operated animal which is much less than half the 

 volume of a normal thyroid. Since only one of these glands 

 was present, the total volume of thyroid tissue was much less 

 than one-fourth of that of a normal animal in the same stage 

 (fig. 71). The structure of this regenerated thyroid was not 

 normal. The gland consists of relatively few large irregular 

 follicles containing colloid. The epithelium (fig. 72) is columnar 

 with oval nuclei and granular cytoplasm. In the cell can be 

 seen droplets of colloid, more numerous than in the normal 

 thyroid (fig. 73). This regenerated gland was located beneath 

 the tongue, considerably anterior to its normal position. This 

 serves as a caution for the use of great care in examining frogs 

 transformed from larvae that have had the thyroid removed. 

 This small regenerated thjToid shown here might easily have 

 been overlooked and the conclusion that the thyroid is not 

 needed for metamorphosis would then have been drawn. 



Another unusual regenerated thyroid is shown in figure 75. 

 This gland is one of two which were normal in structure but 

 large in size. The size of the thyroids of the corresponding 

 control larva is indicated in figure 74. It is interesting to note 

 that the larva with this large regenerated thyroid metamorphosed 

 at a size of about one-half normal and was transformed into 

 a frog of one-third the normal size. The total actual volume 

 of this thyroid was not as great as that of the large control larvae 



