128 W. W. SWINGLE. 



fresh thymus gland to the larvae of Rana pipiens has no effect 

 upon the growth and metamorphosis of these animals, and the 

 gonads appear to be unaffected by such feeding. 



PART II. EXPERIMENTS WITH FEEDING THYMUS POWDER. 



The following experiment was started May 3, 1915: The 

 larvae of Rana pipiens were used for the first half of the work. 

 These larvae had been reared from the egg in the laboratory and 

 were all from one batch of eggs. The feeding work was begun 

 when the tadpoles averaged 28 mm. in length. Most of the 

 larvae had tiny limb buds of the posterior extremities, at this 

 time, though, as yet, the buds had not differentiated into their 

 two primary divisions. Two series of animals were used, one 

 for feeding work, the other for controls. Both groups were kept 

 under uniform conditions of temperature, light, and water supply. 

 Finger bowls were used as containers, five larvae to a bowl. Tap 

 water was used to cover them and was changed daily. 



The animals of the control group were, each day, fed fresh 

 water algae, and dried flour paste. This same food was also 

 given to the thymus-fed larvae, though in smaller amounts in 

 order to obviate any possibility of inanition. 



The thymus used was the desiccated glands of sheep prepared 

 by Armour & Co. This powder was mixed with wheat flour 

 in the proportions of three grams of thymus to five grams of 

 flour, enough water was added to make a thick paste which 

 was allowed to dry at room temperature. Pieces of the dried 

 thymus paste about the size of one's thumb nail were finely 

 crumbled and fed to the larvae. The mixture seemed palatable 

 to the tadpoles for they ate it greedily. The same amount of 

 paste was given each day. 



The control animals grew rapidly, and in a few days the limb 

 buds had differentiated into small legs. The development of the 

 thymus-fed larvae was somewhat slower and, in regard to length, 

 were two or three millimeters behind their controls. On the 

 whole, however, growth in the two series of larvae was approxi- 

 mately equal, when the fact is considered that the thymus-fed 

 larvae received less algae than the controls. 



The limbs of the thymus-fed tadpoles differentiated somewhat 



