This year I had a considerable number of tadpoles of the common English frog, which were hatched between March 26 and March 29. On April 12, when they had all passed the stage of external gills and developed internal gills and opercula, I divided them into two lots, one in a shallow pie-dish, the other in a glass cylinder. To one lot I gave a portion of rabbit's thyroid, to the other a piece of rabbit's liver. They fed eagerly on both. Afterwards I obtained at intervals of a week or so the thyroid of a sheep. I have seen no precise details of Gudernatsch's method of feeding tadpoles, but my own method was simply to put a piece of thyroid into the water containing the tadpoles and leave it there for several days, then to take it out and put in another piece, changing the water when it seemed to be getting foul. April 22. Noticed that the non-thyroid tadpoles were larger than those fed on thyroid. Changed the former into the pie-dish and the latter into the glass jar, to make sure that the difference in size was not due to larger space. May 3. Only eighteen of the non-thyroid tadpoles surviving, owing to the water having become foul, but these are three times as large as those fed on thyroid. In the latter no trace of hind-legs was visible, but the abdominal region was much emaciated and contracted, while the head region was broader. May 4. Noticed minute white buds of hind-legs in the thyroid-fed tadpoles. May 6. A number of the thyroid-fed were dying, and the skin and opercular membranes were swollen out away from the tissues beneath. Largest normal tadpole, 2.7 cm. long. body, 1.0 " tail, 1.7 " Largest thyroid-fed tadpole, 1.1 cm. long. body, 0.5 " tail, 0.6 " May 10. A great number of the thyroid-fed dead and the rest dying, lying at the bottom motionless. They now had the tail much shorter, and the fore-legs showing as well as the hind, but the latter not very long, and without joints or toes. Period from first feeding with thyroid, thirty days. I now decided to feed the controls with thyroid, expecting that as they were large and vigorous they would have strength enough to complete the metamorphosis and become frogs. May 15. Fed the controls with thyroid for first time. The smallest of them was in total length 1.7 cm. body, 0.7 " tail, 1.0 " The largest measured was in total length 2.2 " body, 0.8 " tail, 1.4 " May 25. All but two of the tadpoles dead. The tails were only half the original length, all had well-developed hind-legs, some with toes, but the fore-legs were beneath the opercula, not projecting from the surface.