400 W. W. SWINGLE 



MATERIAL AND METHODS 



Late in March (1918) several bunches of Rana pipiens eggs in 

 late segmentation stages were collected and brought to the lab- 

 oratory to develop. All of the animals used in a single experi- 

 ment were taken from cultures, the larvae of which came orig- 

 inally from the same bunch of eggs, hence were of the same 

 age. The animals were separated into lots of fifty each when 

 the free feeding stage was reached and kept in large glass con- 

 tainers under identical environmental conditions. Several groups 

 of fifty tadpoles each were fed inorganic iodin crystals finely 

 ground, mixed with wheat flour in the proportions of 1 to 100, 

 using the same procedure described by the writer in a previous 

 paper ('18) for administration of thyroid extract. Another lot 

 of larvae was fed iodoform mixed with flour as described. Two 

 other cultures were fed potassium iodide, also in flour, using the 

 same proportions as before. Solutions of potassium iodide were 

 prepared and an attempt made to rear the tadpoles in them, 

 but the method proved unsatisfactory and so was abandoned. 



The larvae were fed each day, care being taken not to overfeed ; 

 the water was changed daily, and with the advent of warm 

 weather twice daily. Very early in the work it was observed 

 that unless the iodin was mixed with food in some way, the 

 larvae refused to eat it. This was especially true of the iodin 

 crystals. Great mortality results if the crystals are merely 

 thrown into the containers among the tadpoles. The quickest 

 and most effective way of rendering the iodin palatable was to 

 mix finely ground crystals with wheat flour (1 to 100) stir until 

 the flour was a delicate brownish hue and then feed the dry 

 mixture. Small bits of algae were fed the animals along with 

 the iodin. 



The various mixtures of iodin and flour prepared by the 

 method described by the writer in a previous paper (T8) for 

 thyroid administration keep well, with the exception of the iodin 

 crystals and flour. The mixture appears to lose strength after 

 about two weeks. Regarding the chemical nature of the sub- 

 stances formed by the mixture of iodin and its compounds with 



