GROWTH OF AMPHIBIA AFTER THYROIDECTOMY 21 



mortem changes that entirely alter their volume and shape. 

 There is no practical method by which the exact volumes of 

 these small organs can be determined. ' One could section them 

 and then calculate the volume of each section by use of a plani- 

 meter, and in this way, except for large errors due to shrinkage, 

 estimate approximately the volume of the organs. However, 

 in the case of most of the organs the variability is so large and 

 the actual differences between the control and thyroidless groups 

 are so slight that the result to be obtained would not justify 

 the effort required* We are certain that only large differences 

 in size of glands of these larvae can be detected by any known 

 practical method, and that any small differences noted are as 

 likely to be due to errors of measurement as to the experiment 

 itself. 



Brain. Figures 22 to 27 show the general character of the 

 brain in the various animals. The brain in the thyroidless 

 animals appears to be relatively small and undifferentiated when 

 compared with the brain of controls of the same age. Com- 

 parison of figures 24 and 25 shows the difference that exists 

 between most of the control and thyroidless animals. In the 

 former the brain is practically the same shape and size as that 

 of the frogs immediately after metamorphosis (fig. 23), whereas 

 the brain of the thyroidless larva, especially in the telencephalon, 

 thalamus, optic lobes, and cerebellum, is shaped very much like 

 that of a very small control larva (fig. 22) . This undeveloped 

 condition of the brain of the thyroidless larvae may be noted in 

 older and larger specimens. Gradually, however, the brain of 

 these specimens tends to become differentiated (fig. 26) and 

 finally it assumes a condition practically the same as that of 

 the young frog (fig. 27) except for the shape of the anterior 

 part of hemispheres. In some of the large thyroidless larvae (fig. 

 26) the optic lobes are relatively short, and of a consequence 

 the cerebellum bulges dorsally and hence appears relatively 

 larger than in the other figures shown here, but the difference 

 in size is apparent rather than real. In the still older large 

 thyroidless larvae (fig. 27) the optic lobes are relatively longer 

 than in the former (fig. 26) and overhang the cerebellum, partially 



