Growth and Metamorphosis of Tadpoles. 



301 



to avoid errors such as might arise in consequence of reduction in the 

 original numbers by a high mortality in any particular dish. 



(16) Finally, an experiment was devised to determine the effect of fats in 

 hastening or retarding the abnormally rapid rate of metamorphosis induced 

 by a large intake of iodine. In this experiment twenty tadpoles were used 

 for each fat to be tested. The diet consisted of a modification of that 

 originally used by Swingle, who produced metamorphosis very rapidly by 

 means of a mixtux'e of one part of metallic iodine to 100 parts of flour. 

 In the present experiment the same proportion of iodine was employed,, 

 but with this difference, that instead of flour, a mixture of flour and 

 caseinogen, in the proportion of eighty-five parts of the former to fifteen 

 parts of the latter, was used. The fats to be tested were added in quantities 

 sufficient to make a soft dough. Pond weed was not given to any of the 

 tadpoles in this series. The fats tested were butter, cocoa-nut oil, oleic 

 acid, linseed oil and cod-liver oil. The observations were controlled by 

 forty tadpoles, twenty of which received the basal diet without fat or iodine, 

 and the remaining twenty the basal diet with iodine but without fat. 



Results of the Experiments. 

 A. Observations on Groivth : — 



(1) The rate of growth of tadpoles receiving an excess of the fats, in an 

 otherwise adequate dietary, was greatly delayed (fig. 1). This retardation of 

 growth was not attributable solely to a lesser food intake, although this may 

 have been responsible in some measure for it, but was dependent on other 

 and unknown causes. The fats tested ranged themselves in the following 

 order with respect to the severity of the retardation of growth induced by 

 them during a period of fifty-three days : arachis oil, cod-liver oil, linseed oil, 

 cocoa-nut oil, oleic acid, lard and butter. 



(2) Iodine in the proportion of 0"5 to 1"0 mgrm. per gramme of food- 

 mixture compensated, in greater or lesser degree, for the retardation of 

 growth induced by certain fats, namely, butter, oleic acid, cocoa-nut oil, lard 

 and arachis oil (figs. 2 and 3). On the other hand, no favourable influence 

 on the rate of growth was exercised by this dosage of iodine in the presence 

 of linseed oil and cod-liver oil ; the rate of growth was, on the contrary, 

 further retarded (fig. 4). The effect of iodine in compensating for the 



'retarded growth induced by fats was due either to its action in increasing 

 the intake of food or to its action in improving food-assimilation, or to both. 

 With respect to this action of iodine, the fats used group themselves into 

 two categories : those in whose presence food-assimilation was favoured by 



