FEEDING THYMUS TO FROG LARV/E. I2Q 



more slowly than the limbs of the controls, and appeared to be 

 smaller in size. There were no observable differences in the 

 pigmentation of the two groups. 



This series of larvae was kept until June 3, just one month 

 from the date the thymus feeding began. They were measured 

 on this date, the control larvae averaging 39.5 mm., the thymus- 

 fed larvae 37.5. The tadpoles were then killed and preserved 

 for miscroscopic examination. It is evident that the thymus 

 feeding did not accelerate growth in the animals of this group. 

 The two millimeters difference between the average length of 

 the control group and the average length of the thymus-fed 

 group is, in animals so variable in their growth capacities as frog 

 larvae, negligible. 



On May 9, a second series of frog larvae were started upon the 

 thymus diet, appropriately controlled. The controls and thy- 

 mus-fed larvae of this series averaged 35 mm. All of the larvae 

 had hind legs though they were very small, and in most of them 

 the toe points were just appearing. These animals were fed 

 the same kind, and amount of food, as the first series received. 



The thymus feeding continued for eighteen days, during this 

 period both controls and thymus-fed larvae grew rapidly, but as 

 recorded for the first series, the controls differentiated limbs 

 faster. 



The length of the two groups of tadpoles was approximately 

 equal, both groups averaging 41.5 mm. eighteen days after the 

 experiment began. 



Two other series of larvae were started later, but the tadpoles 

 were too far along in metamorphosis. No results worth recording 

 were obtained. 



August, 1915, I obtained some larvae of the common bull-frog, 

 Rana catesbiana. When found, these tadpoles averaged 32 mm. 

 in length. All except six were used for another experiment. 

 The six remaining larvae were divided into lots of three each, 

 one for controls, the other for thymus feeding. The nature of 

 the food, and amount fed was the same as that recorded for earlier 

 experiments. 



For twenty-one days, this series of tadpoles was fed thymus, 

 and at the end of that period, there was no appreciable difference 



