32 E. R. HOSKINS AND M. M. HOSKINS 



that the thymus is relatively larger in the thyroidless larvae, 

 than in control larvae just about ready to undergo metamorphosis. 



Microscopic structure. The thymus consists of a medulla of 

 varying proportions and a cortex. There is to be seen no con- 

 stant histological difference between the thymus of control and 

 thyroidless larvae at the time of metamorphosis (figs. 64 to 67). 

 The cortex consists almost entirely of small lymphocytes. The 

 medulla contains both small lymphocytes and large thymic cells 

 with large and often irregular nuclei and granular eosinophilic 

 cytoplasm. The percentage of large cells varies in thyroidless 

 larvae of different ages and in the control larvae and frogs ; there 

 is no marked difference noticeable in this respect between the 

 thymi of the various groups of animals. 



During metamorphosis the entire thymus shrinks in volume, 

 but does not appear to be changed in structure. The cells are 

 not smaller after metamorphosis than before. In some speci- 

 mens the cells are placed more closely together than before, 

 but in other cases this is not so. In no case can this account 

 for the entire shrinkage of the organ, and hence the shrinkage 

 is not due merely to loss of fluids. 



Epithelioid (parathyroid) bodies. The epithelioid bodies of the 

 control and thyroidless larvae of the same size are themselves 

 the same size within limits of normal variability, as shown by 

 comparison of camera-lucida drawings and direct comparison of 

 the organs themselves. Figures 50 and 51 seem to show that 

 the volume of the epithelioid bodies of the experimental larvae 

 are slightly larger than those of the control larvae of the same 

 size, but in other specimens the reverse is true. In larvae these 

 bodies are usually spheroid in shape and their volume can be 

 measured approximately with the formula 4/3 tt,*. This was 

 done, but the measurements do not show any constant difference 

 in size between the epithelioid bodies of thyroidless and control 

 larvae of the same size. 



During metamorphosis there is practically no change in the 

 absolute size of the epithelioid bodies, except possibly a slight 

 decrease of the average, but since the volume of the animal 

 decreases at this time, as stated above, the relative size of the 

 epithelioid bodies is increased considerably. 



