GROWTH OF AMPHIBIA AFTER THYROIDECTOMY 5 



larvae which were sectioned showed any thyroid tissue, it is prob- 

 able that the operation was successful in most if not all cases. 



The hypophysis (figs. 3 and 4) three months after the operation 

 exhibited no peculiarities of structure in the thyroidless animals 

 that could be attributed to the operation. Micrometric deter- 

 minations averaged practically the same as in the control animals 

 when allowance was made for the size of the animals in each 

 case. The hypophysis shown in figure 3 (control) is smaller 

 than that in figure 4 (operated), but the entire animal was also 

 smaller. The cells of the hypophysis are arranged in clusters 

 in both larvae, but there is no other evidence of functional 

 differentiation. 



R. sijlvatica~l 916-1 918 



A comparison of figures 6 and 7 of R. sylvatica of 6.5 mm. 

 in length shows the longitudinal dimension of the area of ento- 

 derm removed in the operation. This included not only the 

 entire anlage of the thyroid gland, but also a considerable portion 

 of the floor of the pharynx and a part of the surrounding mesen- 

 chyma. The anlage in this stage is comparatively large. It ex- 

 tends caudally and ventrally to the pericardium, but with care it 

 may be removed without injury to the latter. Blood-vessels re- 

 moved during the operation usually regenerated. The hypophysis 

 (fig. 6) is already ventral to the infundibulum. It does not appear 

 in figure 7 because of the orientation of the specimen for sectioning. 



Figures 8 and 9 of larvae four days after the operation show 

 that the floor of the pharynx and mouth have been completely 

 regenerated. The thyroid which is well developed in the control 

 animal (fig. 9) is entirely missing from the operated larva 

 (fig. 8X). The hypophysis is well formed in both specimens 

 and shows no difference in size or structure at this time, although 

 in older thyi'oidless tadpoles it is found to have undergone hyper- 

 trophy. No other changes in the organs are noticeable until 

 the time of metamorphosis. 



In the larva represented in figure 8 and in many other speci- 

 mens the removal of mesoderm with the thyroid anlage had no 

 effect on the development of the muscles and cartilages of this 



