FEEDING EXPERIMENTS ON TADPOLES 457 



early days of the experiment than the tadpoles fed on testicle, 

 ovarj'^ and adrenal. The latter three groups also ran similar 

 courses. The set fed on plants showed a rather rapid growth, 

 about equal to that of the spleen, liver and thymus groups. The 

 tadpoles in these groups reached on the average a length of 30 

 to 31 mm at the time of metamorphosis, while the specimens 

 in the slower growing groups reached a length of 33 to 34 mm 

 before they metamorphosed. This fact might lead one to con- 

 clude that the materials fed to the faster groups, vegetable, blood, 

 spleen, liver and thymus, contain an agens which causes a more 

 rapid differentiation than the foods given to the second groups, 

 testicle, ovary, adrenal. This, however, is not the case, since in 

 the first group the spleen- and thymus-fed tadpoles exhibited no 

 signs of an approaching metamorphosis at a time when the tad- 

 poles of the second group had begun to leave "the water. 



Attention should be called to the fact that in this experiment 

 as in the previous ones of 1912 the spleen- and thymus-fed groups 

 (spleen had not been fed in 1911) showed almost parallel courses 

 of development, neither reaching the stage of metamorphosis 

 when the other groups had. Whether or not this indicates an 

 influence of the spleen, or perhaps of all lymphatic organs, simi- 

 lar to that of the thymus in retarding differentiation I am at 

 present unable to decide. 



The tadpoles intended for a pancreas diet did not feed freely 

 and most of them died early. 



The starved animals did not differentiate beyond their original 

 stage and began to die about 4 weeks after the beginning of the 

 experiment. 



The pigmentation exhibited by the animals was very much the 

 same as in previous experiments. The thymus-fed mdividuals, 

 however, did not become as dark as usual. Those fed on blood 

 and plants became very dark. 



Experiment VI 



Rana temporaria, Set V to VII. Figure 5, a to e. Probably three 

 sets mixed. They hatched April 6 to 8 and were not fed until ]\Iay 

 24, when the experiment began. 



