4 E. R. HOSKINS AND M. M. HOSKINS 



RESULTS 



Amhly stoma — 1 91 6 



Growth. There were no constant external changes evident 

 among the thyroidectomized Amblystoma larvae in the three 

 months after the operation during which the animals were under 

 observation. At the end of this time all those surviving were 

 killed for microscopic examination. The gills and legs developed 

 normally. The average length of the seven surviving controls 

 at this time was less than that of the seven experimental larvae, 

 but the number of specimens was too small for final conclusions 

 with regard to growth rate, especially since the largest of the 

 fourteen was one from which the thyroid had been removed. 

 The largest thyroidless larva was 30 mm. long and the largest 

 control 24 mm. One probable source of variation in growth 

 was the food supply, as the animals were isolated and may very 

 likely have had unequal amounts of food. 



Microscopic changes. Four days after the operation the ani- 

 mals had attained the length of 12.5 mm. Sections of control 

 animals show that the thyroid at this time is long and narrow, 

 lying ventral to the pharynx and anterior to the heart. In 

 the animals from which the thyroid had been removed the floor 

 of the pharynx healed without regeneration of the gland. The 

 hypophysis was not structurally different from that of the con- 

 trols in this stage. 



Three months after the operation there were still no differences 

 to be observed between sections of the control and experimental 

 Amblystoma larvae, except of course absence of the thyroid 

 from the latter. The cartilages and muscles ventral to the 

 pharynx developed normally despite the removal of mesoderm 

 in this region. The thyroid glands, which were lacking in all 

 the experimental animals were well developed in the controls at 

 this time (figs. 1, 2, and 5). The vesicles are found scattered 

 along the ventral aortae. Colloid is abundant, although the cells 

 have very little cytoplasm, the wall of the follicle being very 

 thin in most cases. The gland is about 0.4 mm. in length and 

 of variable shape. Since none of the thirteen experimental 



