36 E. R. HOSKINS AND M. M. HOSKINS 



at the time of metamorphosis, but in proportion to the size of 

 the animal it was several times larger than normal (compare 

 figs. 71, 74, and 75), and this fact probably accounts for the 

 precocious metamorphosis similar to that which Gudernatsch 

 ('12) obtained by feeding thyroid. 



Spleen. The variability in the size of the spleen in amphibia 

 as in manamals is very great and seems to follow no definite 

 laws. In our normal larvae and frogs the variability of this 

 organ was fully 100 per cent. The 'average' size of the spleen 

 in the control larvae just before metamorphosis was about the 

 same as that of the thyroidless larvae of the same size (figs. 79 

 and 80). During metamorphosis the spleen changes little, if 

 any, in absolute size except perhaps a very slight increase; but 

 its relative size increases in about the inverse ratio as the change 

 in the size of the body, which, as stated above, decreases by one- 

 half to two-thirds. The proportions of the spleen in the larger 

 thyroidless larvae are about the same as in the control larvae 

 and hence somewhat less than in young frogs, but, as shown in 

 figures 83, 84, 86, and 92, the variabihty is very large. 



The spleen becomes differentiated in structure by the time 

 the larva is 26 nom. long or even before this time (fig. 76). The 

 cellular structure consists of a dense reticulum containing small 

 lymphoctyes and large splenic cells with more cytoplasm than 

 the lymphocytes. The nuclei of the large cells are usually 

 lightly stained. The nuclei in the spleen are often irregular in 

 outline and many seem to be dividing by simple fission. 



There is no particular cellular difference between the spleens 

 of control and thyroidless larvae (figs. 77 and 78). 



Kidneys. The kidneys in our various groups of animals re- 

 semble in general the liver and the heart in changes of relative 

 size. Given a control and thyroidless larva of the same size 

 (figs. 79 and 80), the kidneys of the two are the same size, within 

 the limits of normal variation. 



During metamorphosis the size of the kidney decreases con- 

 siderably, but the decrease is proportionally less than the loss 

 in the size of the entire animal, so that a young frog has relatively 

 a larger kidney than a normal tadpole (figs. 79 and 81). 



