230 W. W. SWINGLE. 



of the hyobranchial region to the symphysis of the mandible. 

 The skin and superficial muscles were pulled apart on either side 

 and pinned down, exposing the geniohyoid and ceratohyoid 

 muscles. The thyroid glands are located in the triangle on each 

 side of the median line, formed by the geniohyoid and ceratohyoid 

 muscles. The large size of the glands and their intimate relation 

 to the blood vessels which traverse the triangle render their 

 location easy. They can be removed from both sides of the 

 animal with but little injury to the blood vessels if first dissected 

 free with a razor-edge needle and gently pulled out with a fine- 

 pointed pair of forceps. Even if the blood-vessels are injured and 

 considerable bleeding occurs, the post-operative effects upon the 

 animal are negligible. It is surprising how much surgical manip- 

 ulation an axolotl can withstand without showing any post- 

 operative symptoms. Following gland removal the skin and 

 superficial muscles are sutured along the median line. The 

 animals quickly recover from the anesthetic and appear to suffer 

 no ill effects from the operation. Food was witheld for four or 

 five days after thyroidectomy in order to prevent any possibility 

 of tearing the sutures while swallowing. 



The thyroidless animals were kept for five months in large 

 concrete tanks through which fresh water ran constantly. The 

 food consisted of worms, insect larva?, and occasionaliy, pieces 

 of fresh liver. 



Five months following thyroid removal, three animals were 

 injected twice, at five day intervals with eighty milligrams of 

 tyrosine in which two atoms of iodine had been substituted for 

 two hydrogen atoms of the molecule, forming the well-known 

 compound 3-5-diiodotyrosine. The animals metamorphosed 

 within seventeen days following the first injection. 



Three control thyroidless axolotls injected with equal quanti- 

 ties of pure tyrosine and 3-5 dibrom tyrosine, i.e., tyrosine in 

 which two bromine atoms had been substituted for two hydro- 

 gens, failed to transform. Later, one of the controls was injected 

 with d third dose of eighty milligrams of dibromtyrosine but with 

 negative results. Dissection of the metamorphosed iodotyrosine- 

 injected animals showed no trace of thyroid tissue present. 



Rogoff and Marine ('17) showed that iodized blood serum 

 accelerates metamorphosis of normal tadpoles, and that the 



