PLATE 6 



EXPLANATION OF FIGURES 



R. sylvatica. Various organs 



64 Thymus of full-grown larva. X 40. 



65 Thymus of thyroidless larva larger, than that of figure 64, but same age. 

 X iO. 



66 Small lymphocytes and large thymic cells from thymus shown in figure 

 64. X 670. 



67 From thymus shown in figure 65. X 670. 



68 Epithelioid body from same larva as figure 64. Cap., capsule. X 670. 



69 Epithelioid body from larva of figure 65. X 670. 



70 Small regenerated thyroid, showing unusual structure. X 160. 



71 Normal thyroid of young frog. The structure and size are the same as 

 before metamorphosis. X 160. 



72 From figure 70. X 670. 



73 From figure 71. X 670. 



74 Thyroid of half-grown normal larva. X 160. 



75 Thyroid of precocious frog (fig. 20). This thyroid regenerated and hy- 

 pertrophied. Compare with figure 74 from a normal larva of the same age. 

 X 160. 



76 Spleen of small normal larva (23 mm.) showing the two types of splenic 

 cells. X 670. 



77 Spleen of control larva at maximum length before metamorphosis. X 

 670. 



78 Spleen of thyroidless larva of same age as that of figure 77. X 670. 



