FEEDING THYMUS TO FROG LARV.E. 13! 



fed groups were measured. The controls averaged 29.5 mm., 

 the thymus-fed larvae 30 mm. The disparity in the average 

 lengths of the two series is negligible. The fact to be noticed 

 is that both experimental and control larvae grew considerably 

 and that both groups possessed tiny limb buds on December 7, 

 when the experiment was discontinued. 



SUMMARY OF THYMUS POWDER EXPERIMENT. 

 To sum up the results obtained in the experiments just recorded 

 it may be said that feeding powdered thymus glands to amphibian 

 larvae of the species Rana pipiens, Rana catesbiana and Rana 

 (species undetermined) does not accelerate growth 

 beyond the normal rate. Some of the observations recorded 

 do, however, indicate that thymus feeding may have a slight 

 inhibitory effect upon limb development. However, in animals 

 so extremely variable in their growth capacities as frog larvae, 

 to attribute such a slight retardation in limb development to the 

 effects of thymus feeding would be rather unsafe. Aside from 

 this extremely slight retardation in limb development of the 

 thymus-fed larvae, no results were obtained which would indicate 

 that the feeding of powdered thymus gland to amphibian larvae 

 has any effect upon growth. 



SUMMARY AND CONCLUSION. 



To sum up the results of this experiment, feeding thymus 

 gland, either in the fresh state or in the powdered form, to frog 

 larvae of the species Rana pipiens, catesbiana, and one undeter- 

 mined species, does not accelerate the growth processes, or retard 

 the metamorphosis of these animals. The tadpoles seem to 

 develop normally in every respect. The gonads do not appear 

 to be effected by thymus feeding. 



These results are at variance with the findings of Gudernatsch. 

 It may be, however, that the species of frog used by this investi- 

 gator reacts differently to thymus feeding than does Rana 

 pipiens and catesbiana, thus accounting for the disparity of the 

 results obtained by this investigator and myself. 



One point, however, must not be overlooked in this connection, 

 i. e., that the variability of the growth rate in frog larvae is 



