410 W. W. SWINGLE 



ments were carried out with these neotenous individuals, in the 

 hope of determining the endocrine locus responsible for the failure 

 to metamorphose at the proper time. The first procedure was to 

 test the physiological activity of the thyroid apparatus of such 

 animals bj^ transplanting the glands into immature larvae of the 

 same species with undifferentiated legs and noting the effect upon 

 metamorphosis. 



December 17, 1920, three immature larvae, averaging 33.5 mm. 

 total length, without hind limbs, except undifferentiated epithe- 

 lial buds, were engrafted with the thyroid glands of 80-mm. 

 neotenous larvae with hind legs 11.6 mm. long. The engrafted 

 forms were the smallest obtainable at this season of the year and 

 not over six months of age. It is unfortunate that a larger number 

 could not have been used, but the results are clean-cut, as will 

 be seen later, when the experiment was rechecked with another 

 group of animals in midsummer. 



One animal was in advanced stages of metamorphosis on 

 January 17th, just one month from date of grafting. The right 

 fore leg was through the skin and the left fore leg appeared the 

 day following (January 18th). For a week previous to the 

 first appearance of the fore limbs the engrafted animal showed 

 marked signs or hyperthyroidism, such as emaciation, protrusion 

 of the eyes, slight tail atrophy, and autolysis of the skin over the 

 region of the fore legs. The control larvae at this time averaged 

 34 to 38 mm. in length and showed no change in regard to leg devel- 

 opment from the condition when the experiment was started. 

 The other two larvae of the engrafted culture developed fore limbs 

 January 22nd and typical frog mouth. All of the animals died 

 before tail resorption was complete. Figure 1 is a drawing of an 

 engrafted larva and its control. Figure 2 shows the neotenous 

 type of tadpole from which the thyroid glands were taken. 

 Generally the limb development is less marked than the drawing 

 indicates. When the drawings were made the animals were 

 nearly a year past their normal time of metamorphosis. 



Briefly summarized, the experiment shows that transplantation 

 of the thyroid glands of neotenous larvae 80 mm. total length, 

 with differentiated hind limbs 11.6 mm., into immature tadpoles 



