• W. W. SWINGLE 



163 



checks. The original plan was to use only distilled water, but water 

 distilled by ordinary methods was found toxic for tadpoles. Only 

 quartz distilling was satisfactory. Unfortunately the laboratory 

 facilities for this method were inadequate for producing 30,000 cc. 

 daily, so tap water was substituted. Princeton water possibly con- 

 tains minute traces of iodine, but if so it is present in such small 

 quantities as to prove negligible in the present experiment. 



The nature and amount of the food fed were carefully controlled, 

 and consisted of fat-free casein, yeast, potato starch, and rolled oats. 

 3 days a week small quantities of Elodea were placed in the con- 

 tainers. Toad larvae do not feed upon the Elodea itself, but only 

 upon the diatoms and other organisms living on it. It is evident, 

 from the choice of foods listed, that the possibility that iodine might 

 be present in minute traces was not absolutely ruled out. However, 

 as in the case of the tap water, if present it was insufficient in quantity 

 to affect metamorphosis during the interval the experiment lasted. 

 The water of the cultures was changed daily and the containers were 

 well rinsed. An m/20 stock solution of iodine was employed. The 

 gram concentration of iodine used for the various cultures is given 

 in the following table. This amount was given daily. 



Frog and toad larvae are said not to swallow water' but to absorb it 

 through the integument, consequently this method of iodine admin- 

 istration is very satisfactory. The obvious defect of this method is 

 that it is impossible to determine the exact amount of water absorbed 

 or iodine obtained by the larvae per day. 



For the sake of brevity, all descriptions of the various cultures will 

 be presented in tabular form. Date of first iodine administration, 

 May 19; average total length 9 mm.; no limb buds present. A plus 

 sign indicates that fore limbs have appeared. 



' Boulenger, E. G., Reptiles and batrachians, London and New York, 1914, xiv, 

 197-199. 



